Effects of Influenza Vaccination in the United States During the 2018-2019 Influenza Season

Influenza Vaccine Effectiveness Against Influenza A-Associated Emergency Department, Urgent Care, and Hospitalization Encounters Among US Adults, 2022-2023

BACKGROUND

The 2022-2023 United States influenza season had unusually early influenza activity with high hospitalization rates. Vaccine-matched A(H3N2) viruses predominated, with lower levels of A(H1N1)pdm09 activity also observed.

METHODS

Using the test-negative design, we evaluated influenza vaccine effectiveness (VE) during the 2022-2023 season against influenza A-associated emergency department/urgent care (ED/UC) visits and hospitalizations from October 2022 to March 2023 among adults (aged ≥18 years) with acute respiratory illness (ARI). VE was estimated by comparing odds of seasonal influenza vaccination among case-patients (influenza A test positive by molecular assay) and controls (influenza test negative), applying inverse-propensity-to-be-vaccinated weights.

RESULTS

The analysis included 85 389 ED/UC ARI encounters (17.0% influenza A positive; 37.8% vaccinated overall) and 19 751 hospitalizations (9.5% influenza A positive; 52.8% vaccinated overall). VE against influenza A-associated ED/UC encounters was 44% (95% confidence interval [CI], 40%-47%) overall and 45% and 41% among adults aged 18-64 and ≥65 years, respectively. VE against influenza A-associated hospitalizations was 35% (95% CI, 27%-43%) overall and 23% and 41% among adults aged 18-64 and ≥65 years, respectively.

CONCLUSIONS

VE was moderate during the 2022-2023 influenza season, a season characterized with increased burden of influenza and co-circulation with other respiratory viruses. Vaccination is likely to substantially reduce morbidity, mortality, and strain on healthcare resources.

A Prediction Model for Identifying Seasonal Influenza Vaccination Uptake Among Children in Wuxi, China: Prospective Observational Study

BACKGROUND

Predicting vaccination behaviors accurately could provide insights for health care professionals to develop targeted interventions.

OBJECTIVE

The aim of this study was to develop predictive models for influenza vaccination behavior among children in China.

METHODS

We obtained data from a prospective observational study in Wuxi, eastern China. The predicted outcome was individual-level vaccine uptake and covariates included sociodemographics of the child and parent, parental vaccine hesitancy, perceptions of convenience to the clinic, satisfaction with clinic services, and willingness to vaccinate. Bayesian networks, logistic regression, least absolute shrinkage and selection operator (LASSO) regression, support vector machine (SVM), naive Bayes (NB), random forest (RF), and decision tree classifiers were used to construct prediction models. Various performance metrics, including area under the receiver operating characteristic curve (AUC), were used to evaluate the predictive performance of the different models. Receiver operating characteristic curves and calibration plots were used to assess model performance.

RESULTS

A total of 2383 participants were included in the study; 83.2% of these children (n=1982) were <5 years old and 6.6% (n=158) had previously received an influenza vaccine. More than half (1356/2383, 56.9%) the parents indicated a willingness to vaccinate their child against influenza. Among the 2383 children, 26.3% (n=627) received influenza vaccination during the 2020-2021 season. Within the training set, the RF model showed the best performance across all metrics. In the validation set, the logistic regression model and NB model had the highest AUC values; the SVM model had the highest precision; the NB model had the highest recall; and the logistic regression model had the highest accuracy, F1 score, and Cohen κ value. The LASSO and logistic regression models were well-calibrated.

CONCLUSIONS

The developed prediction model can be used to quantify the uptake of seasonal influenza vaccination for children in China. The stepwise logistic regression model may be better suited for prediction purposes.

Characteristics and evolution of hemagglutinin and neuraminidase genes of Influenza A(H3N2) viruses in Thailand during 2015 to 2018

Background

Influenza A(H3N2) virus evolves continuously. Its hemagglutinin (HA) and neuraminidase (NA) genes have high genetic variation due to the antigenic drift. This study aimed to investigate the characteristics and evolution of HA and NA genes of the influenza A(H3N2) virus in Thailand.

Methods

Influenza A positive respiratory samples from 2015 to 2018 were subtyped by multiplex real-time RT-PCR. Full-length HA and NA genes from the positive samples of influenza A(H3N2) were amplified and sequenced. Phylogenetic analysis with the maximum likelihood method was used to investigate the evolution of the virus compared with the WHO-recommended influenza vaccine strain. Homology modeling and N-glycosylation site prediction were also performed.

Results

Out of 443 samples, 147 (33.18%) were A(H1N1)pdm09 and 296 (66.82%) were A(H3N2). The A(H3N2) viruses circulating in 2015 were clade 3C.2a whereas sub-clade 3C.2a1 and 3C.2a2 dominated in 2016-2017 and 2018, respectively. Amino acid substitutions were found in all antigenic sites A, B, C, D, and E of HA but the majority of the substitutions were located at antigenic sites A and B. The S245N and N329S substitutions in the NA gene affect the N-glycosylation. None of the mutations associated with resistance to NA inhibitors were observed. Mean evolutionary rates of the HA and NA genes were 3.47 × 10 -3 and 2.98 × 10-3 substitutions per site per year.

Conclusion

The influenza A(H3N2) virus is very genetically diverse and is always evolving to evade host defenses. The HA and NA gene features including the evolutionary rate of the influenza A(H3N2) viruses that were circulating in Thailand between 2015 and 2018 are described. This information is useful for monitoring the genetic characteristics and evolution in HA and NA genes of influenza A(H3N2) virus in Thailand which is crucial for predicting the influenza vaccine strains resulting in high vaccine effectiveness.

Dynamic Patterns and Predominance of Respiratory Pathogens Post-COVID-19: Insights from a Two-Year Analysis

INTRODUCTION

Respiratory tract infections (RTIs) stand out as the most frequent causes leading to visits to the emergency department and hospitalizations. This study aims to assess the types and prevalence of respiratory infections across two years following the end of the COVID-19 pandemic.

METHODS

Patients presenting with an influenza-like illness (ILI) were tested using multiplex RT-PCR (QIAstat-Dx, Qiagen). The multiplexed RT- PCR test detects 21 respiratory viruses and bacteria.

RESULTS

During the study period, PCR test was done on a total of 1,790 samples were tested, and 712 (40%) were positive for a total of 796 pathogens. The mean age (± SD) of the participants was 20.1 ± 28.4 years in 2022 and 21.9 ± 27.6 years in 2023. Among the detected pathogens, the most prevalent were Rhinovirus/Enterovirus 222 (12.4%), followed by RSV A&B (103 cases, 5.7%), and H1N1 Influenza (77 cases, 4.3%). Additionally,  Influenza A/B constituted 172 (9.6%) while parainfluenza constituted (58, 3.2%). SARS-CoV-2 was identified in 3.97% of the samples. Over the two-year period, the monthly pattern of the identified pathogens exhibited fluctuations in the prevalence. Furthermore, variations were observed in the detected pathogens across different age groups.

CONCLUSION

In addition to adding significant knowledge to the field of respiratory viral infections, this study emphasizes the necessity of ongoing research and surveillance for the detection and characterization of respiratory viruses, particularly those with the potential for emergence. Such studies would also require setting up a strategy for genotyping and/or sequencing of viruses.

Preliminary findings from the Dynamics of the Immune Responses to Repeat Influenza Vaccination Exposures (DRIVE I) Study: a Randomized Controlled Trial

Background

Studies have reported that repeated annual vaccination may influence the effectiveness of the influenza vaccination in the current season. The mechanisms underlying these differences are unclear but might include "focusing" of the adaptive immune response to older strains.

Methods

We established a 5-year randomized placebo-controlled trial of repeated influenza vaccination (Flublok, Sanofi Pasteur) in adults 18-45 years of age. Participants were randomized equally between five groups, with planned annual receipt of vaccination (V) or saline placebo (P) as follows: P-P-P-P-V, P-P-P-V-V, P-P-V-V-V, P-V-V-V-V, or V-V-V-VV. Serum samples were collected each year just before vaccination and after 30 and 182 days. A subset of sera were tested by hemagglutination inhibition assays, focus reduction neutralization tests and enzyme-linked immunosorbent assays against vaccine strains.

Results

From 23 October 2020 through 11 March 2021 we enrolled and randomized 447 adults. We selected sera from 95 participants at five timepoints from the first two study years for testing. Among vaccinated individuals, antibody titers increased between days 0 and 30 against each of the vaccine strains, with substantial increases for first-time vaccinees and smaller increases for repeat vaccinees, who had higher pre-vaccination titers in year 2. There were statistically significant reductions in the proportion of participants achieving a four-fold greater rise in antibody titer for the repeat vaccinees for A(H1N1), B/Victoria and B/Yamagata, but not for influenza A(H3N2). There were no statistically significant differences between groups in geometric mean titers at day 30 or the proportions of participants with antibody titers ≥40 at day 30 for any of the vaccine strains.

Conclusions

In the first two years, repeat vaccinees and first-time vaccinees had similar post-vaccination geometric mean titers to all four vaccine strains, indicative of similar levels of clinical protection. The vaccine strains of A(H1N1) and A(H3N2) were updated in year 2, providing an opportunity to explore antigenic distances between those strains in humans in subsequent years.

Optimizing Diagnosis and Management of Community-acquired Pneumonia in the Emergency Department

Pneumonia is split into 3 diagnostic categories: community-acquired pneumonia (CAP), health care-associated pneumonia, and ventilator-associated pneumonia. This classification scheme is driven not only by the location of infection onset but also by the predominant associated causal microorganisms. Pneumonia is diagnosed in over 1.5 million US emergency department visits annually (1.2% of all visits), and most pneumonia diagnosed by emergency physicians is CAP.

The association between influenza vaccination uptake and influenza and pneumonia-associated deaths in the United States

BACKGROUND

The influenza mortality burden has remained substantial in the United States (US) despite relatively high levels of influenza vaccine uptake. This has led to questions regarding the effectiveness of the program against this outcome, particularly in the elderly. The aim of this evaluation was to develop and explore a new approach to estimating the population-level effect of influenza vaccination uptake on pneumonia and influenza (P&I) associated deaths.

METHODS

Using publicly available data we examined the association between state-level influenza vaccination and all-age P&I associated deaths in the US from the 2013-2014 influenza season to the 2018-2019 season. In the main model, we evaluated influenza vaccine uptake in all those age 6 months and older. We used a mixed-effects regression analysis with generalised least squares estimation to account for within state correlation in P&I mortality.

RESULTS

From 2013-2014 through 2018-2019, the total number of all-age P&I related deaths during the influenza seasons was 480,111. The mean overall cumulative influenza vaccine uptake (age 6 months and older) across the states and years considered was 46.7%, with higher uptake (64.8%) observed in those aged ≥ 65 years. We found that overall influenza vaccine uptake (6 months and older) had a statistically significant protective association with the P&I death rate. This translated to a 0.33 (95% CI: 0.20, 0.47) per 100,000 population reduction in P&I deaths in the influenza season per 1% increase in overall influenza vaccine uptake.

DISCUSSION

These results using a population-level statistical approach provide additional support for the overall effectiveness of the US influenza vaccination program. This reassurance is critical given the importance of ensuring confidence in this life saving program. Future research is needed to expand on our approach using more refined data.

mRNA vaccines encoding computationally optimized hemagglutinin elicit protective antibodies against future antigenically drifted H1N1 and H3N2 influenza viruses isolated between 2018-2020

Background

The implementation of mRNA vaccines against COVID-19 has successfully validated the safety and efficacy of the platform, while at the same time revealing the potential for their applications against other infectious diseases. Traditional seasonal influenza vaccines often induce strain specific antibody responses that offer limited protection against antigenically drifted viruses, leading to reduced vaccine efficacy. Modern advances in viral surveillance and sequencing have led to the development of in-silico methodologies for generating computationally optimized broadly reactive antigens (COBRAs) to improve seasonal influenza vaccines.

Methods

In this study, immunologically naïve mice were intramuscularly vaccinated with mRNA encoding H1 and H3 COBRA hemagglutinins (HA) or wild-type (WT) influenza HAs encapsulated in lipid nanoparticles (LNPs).

Results

Mice vaccinated with H1 and H3 COBRA HA-encoding mRNA vaccines generated robust neutralizing serum antibody responses against more antigenically distinct contemporary and future drifted H1N1 and H3N2 influenza strains than those vaccinated with WT H1 and H3 HA-encoding mRNA vaccines. The H1 and H3 COBRA HA-encoding mRNA vaccines also prevented influenza illness, including severe disease in the mouse model against H1N1 and H3N2 viruses.

Conclusions

This study highlights the potential benefits of combining universal influenza antigen design technology with modern vaccine delivery platforms and exhibits how these vaccines can be advantageous over traditional WT vaccine antigens at eliciting superior protective antibody responses against a broader number of influenza virus isolates.

Interim Estimates of 2023-24 Seasonal Influenza Vaccine Effectiveness - United States

In the United States, annual influenza vaccination is recommended for all persons aged ≥6 months. Using data from four vaccine effectiveness (VE) networks during the 2023-24 influenza season, interim influenza VE was estimated among patients aged ≥6 months with acute respiratory illness-associated medical encounters using a test-negative case-control study design. Among children and adolescents aged 6 months-17 years, VE against influenza-associated outpatient visits ranged from 59% to 67% and against influenza-associated hospitalization ranged from 52% to 61%. Among adults aged ≥18 years, VE against influenza-associated outpatient visits ranged from 33% to 49% and against hospitalization from 41% to 44%. VE against influenza A ranged from 46% to 59% for children and adolescents and from 27% to 46% for adults across settings. VE against influenza B ranged from 64% to 89% for pediatric patients in outpatient settings and from 60% to 78% for all adults across settings. These findings demonstrate that the 2023-24 seasonal influenza vaccine is effective at reducing the risk for medically attended influenza virus infection. CDC recommends that all persons aged ≥6 months who have not yet been vaccinated this season get vaccinated while influenza circulates locally.

PREDAC-CNN: predicting antigenic clusters of seasonal influenza A viruses with convolutional neural network

Vaccination stands as the most effective and economical strategy for prevention and control of influenza. The primary target of neutralizing antibodies is the surface antigen hemagglutinin (HA). However, ongoing mutations in the HA sequence result in antigenic drift. The success of a vaccine is contingent on its antigenic congruence with circulating strains. Thus, predicting antigenic variants and deducing antigenic clusters of influenza viruses are pivotal for recommendation of vaccine strains. The antigenicity of influenza A viruses is determined by the interplay of amino acids in the HA1 sequence. In this study, we exploit the ability of convolutional neural networks (CNNs) to extract spatial feature representations in the convolutional layers, which can discern interactions between amino acid sites. We introduce PREDAC-CNN, a model designed to track antigenic evolution of seasonal influenza A viruses. Accessible at http://predac-cnn.cloudna.cn, PREDAC-CNN formulates a spatially oriented representation of the HA1 sequence, optimized for the convolutional framework. It effectively probes interactions among amino acid sites in the HA1 sequence. Also, PREDAC-CNN focuses exclusively on physicochemical attributes crucial for the antigenicity of influenza viruses, thereby eliminating unnecessary amino acid embeddings. Together, PREDAC-CNN is adept at capturing interactions of amino acid sites within the HA1 sequence and examining the collective impact of point mutations on antigenic variation. Through 5-fold cross-validation and retrospective testing, PREDAC-CNN has shown superior performance in predicting antigenic variants compared to its counterparts. Additionally, PREDAC-CNN has been instrumental in identifying predominant antigenic clusters for A/H3N2 (1968-2023) and A/H1N1 (1977-2023) viruses, significantly aiding in vaccine strain recommendation.

Redirecting antibody responses from egg-adapted epitopes following repeat vaccination with recombinant or cell culture-based versus egg-based influenza vaccines

Repeat vaccination with egg-based influenza vaccines could preferentially boost antibodies targeting the egg-adapted epitopes and reduce immunogenicity to circulating viruses. In this randomized trial (Clinicaltrials.gov: NCT03722589), sera pre- and post-vaccination with quadrivalent inactivated egg-based (IIV4), cell culture-based (ccIIV4), and recombinant (RIV4) influenza vaccines were collected from healthcare personnel (18-64 years) in 2018-19 (N = 723) and 2019-20 (N = 684) influenza seasons. We performed an exploratory analysis. Vaccine egg-adapted changes had the most impact on A(H3N2) immunogenicity. In year 1, RIV4 induced higher neutralizing and total HA head binding antibodies to cell- A(H3N2) virus than ccIIV4 and IIV4. In year 2, among the 7 repeat vaccination arms (IIV4-IIV4, IIV4-ccIIV4, IIV4-RIV4, RIV4-ccIIV4, RIV4-RIV4, ccIIV4-ccIIV4 and ccIIV4-RIV4), repeat vaccination with either RIV4 or ccIIV4 further improved antibody responses to circulating viruses with decreased neutralizing antibody egg/cell ratio. RIV4 also had higher post-vaccination A(H1N1)pdm09 and A(H3N2) HA stalk antibodies in year 1, but there was no significant difference in HA stalk antibody fold rise among vaccine groups in either year 1 or year 2. Multiple seasons of non-egg-based vaccination may be needed to redirect antibody responses from immune memory to egg-adapted epitopes and re-focus the immune responses towards epitopes on the circulating viruses to improve vaccine effectiveness.

Trends in influenza vaccine uptake before and during the COVID-19 pandemic in the USA

OBJECTIVES

To investigate trends of influenza vaccine uptake before and during the COVID-19 pandemic in the USA and explore the associated factors.

STUDY DESIGN

This was a cross-sectional study.

METHODS

Using self-reported data from the National Health Interview Survey during 2014-2021 (response rates ranging from 50.7 to 70.1%), we estimated influenza vaccine uptake. Log-binomial regression models were used to test uptake changes with adjustment for and stratification by demographic and health factors.

RESULTS

We included 58,249 children (mean age: 8.7 years; male: 51.1%) and 205,034 adults (mean age: 47.6 years; male: 48.2%). The prevalence ratio (PR) of uptake change comparing the intra- (2020-2021) to the pre-COVID-19 period (2014-2019) was 0.72 among children, with a 10.7% reduction. Uptake changes were found across subgroups, with higher reduction among those aged 0-2 years, non-Hispanic Black and Hispanic ethnicity, from South and West regions, and with lower household income. For adults, uptake increased before and during COVID-19 (PR = 1.15, 95% confidence interval [CI]: 1.12-1.18) but a 2.3% reduction was found among healthcare personnel (PR = 0.95, 95% CI: 0.90-0.997).

CONCLUSIONS

Influenza vaccination decreased during the COVID-19 pandemic among children and healthcare personnel. Structure inequality to influenza vaccination warrants measures to improve vaccine uptake among vulnerable groups.

High-Dose Influenza Vaccine Is Associated With Reduced Mortality Among Older Adults With Breakthrough Influenza Even When There Is Poor Vaccine-Strain Match

BACKGROUND

High-dose (HD) influenza vaccine offers improved protection from influenza virus infection among older adults compared with standard-dose (SD) vaccine. Here, we explored whether HD vaccine attenuates disease severity among older adults with breakthrough influenza.

METHODS

This was a retrospective cohort study of US claims data for influenza seasons 2016-2017, 2017-2018, and 2018-2019, defined as 1 October through 30 April, among adults aged ≥65 years. After adjusting the different cohorts for the probability of vaccination conditional on patients' characteristics, we compared 30-day mortality rate post-influenza among older adults who experienced breakthrough infection after receipt of HD or SD influenza vaccines and among those not vaccinated (NV).

RESULTS

We evaluated 44 456 influenza cases: 23 109 (52%) were unvaccinated, 15 037 (33.8%) received HD vaccine, and 6310 (14.2%) received SD vaccine. Significant reductions in mortality rates among breakthrough cases were observed across all 3 seasons for HD vs NV, ranging from 17% to 29% reductions. A significant mortality reduction of 25% was associated with SD vaccination vs NV in the 2016-2017 season when there was a good match between circulating influenza viruses and selected vaccine strains. When comparing HD vs SD cohorts, mortality reductions were higher among those who received HD in the last 2 seasons when mismatch between vaccine strains and circulating H3N2 viruses was documented, albeit not significant.

CONCLUSIONS

HD vaccination was associated with lower post-influenza mortality among older adults with breakthrough influenza, even during seasons when antigenically drifted H3N2 circulated. Improved understanding of the impact of different vaccines on attenuating disease severity is warranted when assessing vaccine policy recommendations.

Recommendations for Prevention and Control of Influenza in Children, 2023-2024

This technical report accompanies the recommendations of the American Academy of Pediatrics for the routine use of influenza vaccine and antiviral medications in the prevention and treatment of influenza in children during the 2023-2024 season. The rationale for the American Academy of Pediatrics recommendation for annual influenza vaccination of all children without medical contraindications starting at 6 months of age is provided. Influenza vaccination is an important strategy for protecting children and the broader community against influenza. This technical report summarizes recent influenza seasons, morbidity and mortality in children, vaccine effectiveness, and vaccination coverage, and provides detailed guidance on vaccine storage, administration, and implementation. The report also provides a brief background on inactivated and live-attenuated influenza vaccines, available vaccines this season, vaccination during pregnancy and breastfeeding, diagnostic testing for influenza, and antiviral medications for treatment and chemoprophylaxis. Strategies to promote vaccine uptake are emphasized.

Coverage and impact of influenza vaccination among children in Minhang District, China, 2013-2020

Background

Young children have a great disease burden and are particularly vulnerable to influenza. This study aimed to assess the direct effect of influenza vaccination among children and to evaluate the indirect benefit of immunizing children.

Methods

The influenza vaccination records for all children born during 2013-2019 in Minhang District and surveillance data for reported influenza cases were obtained from the Minhang CDC. 17,905 children were recorded in the vaccination system and included in this study. Descriptive epidemiology methods were used for data analysis, including an ecological approach to estimate the number of influenza cases averted by vaccination and linear regression to estimate the reduction in influenza cases in the general population per thousand additional childhood vaccination doses.

Results

During the study period, the annual vaccination coverage rate ranged from 10.40% in 2013-2014 to 27.62% in 2015-2016. The estimated number of influenza cases averted by vaccination ranged from a low of 0.28 (range: 0.23-0.34) during 2013-2014 (PF: 6.15%, range: 5.11-7.38%) to a high of 15.34 (range: 12.38-18.51) during 2017-2018 (PF: 16.54%, range: 13.79-19.30%). When increasing vaccination coverage rate by 10% in each town/street, a ratio of 7.27-10.69% cases could be further averted on the basis of observed cases. In four selected periods, the number of influenza cases in the general population was most significantly correlated with the cumulative childhood vaccination doses in the prior 2-5 months, and the reduction in influenza cases ranged from 0.73 to 3.18 cases per thousand additional childhood vaccination doses.

Conclusion

Influenza vaccination among children is estimated to have direct effects in terms of averted cases and might provide an underlying indirect benefit to the general population. Vaccination coverage in high-coverage areas should be further expanded to avert more influenza cases.

Development of PREDAC-H1pdm to model the antigenic evolution of influenza A/(H1N1) pdm09 viruses

The Influenza A (H1N1) pdm09 virus caused a global pandemic in 2009 and has circulated seasonally ever since. As the continual genetic evolution of hemagglutinin in this virus leads to antigenic drift, rapid identification of antigenic variants and characterization of the antigenic evolution are needed. In this study, we developed PREDAC-H1pdm, a model to predict antigenic relationships between H1N1pdm viruses and identify antigenic clusters for post-2009 pandemic H1N1 strains. Our model performed well in predicting antigenic variants, which was helpful in influenza surveillance. By mapping the antigenic clusters for H1N1pdm, we found that substitutions on the Sa epitope were common for H1N1pdm, whereas for the former seasonal H1N1, substitutions on the Sb epitope were more common in antigenic evolution. Additionally, the localized epidemic pattern of H1N1pdm was more obvious than that of the former seasonal H1N1, which could make vaccine recommendation more sophisticated. Overall, the antigenic relationship prediction model we developed provides a rapid determination method for identifying antigenic variants, and the further analysis of evolutionary and epidemic characteristics can facilitate vaccine recommendations and influenza surveillance for H1N1pdm.

Age Differences in Comorbidities, Presenting Symptoms, and Outcomes of Influenza Illness Requiring Hospitalization: A Worldwide Perspective From the Global Influenza Hospital Surveillance Network

Background

The Global Influenza Hospital Surveillance Network (GIHSN) was established in 2012 to conduct coordinated worldwide influenza surveillance. In this study, we describe underlying comorbidities, symptoms, and outcomes in patients hospitalized with influenza.

Methods

Between November 2018 and October 2019, GIHSN included 19 sites in 18 countries using a standardized surveillance protocol. Influenza infection was laboratory-confirmed with reverse-transcription polymerase chain reaction. A multivariate logistic regression model was utilized to analyze the extent to which various risk factors predict severe outcomes.

Results

Of 16 022 enrolled patients, 21.9% had laboratory-confirmed influenza; 49.2% of influenza cases were A/H1N1pdm09. Fever and cough were the most common symptoms, although they decreased with age (P < .001). Shortness of breath was uncommon among those <50 years but increased with age (P < .001). Middle and older age and history of underlying diabetes or chronic obstructive pulmonary disease were associated with increased odds of death and intensive care unit (ICU) admission, and male sex and influenza vaccination were associated with lower odds. The ICU admissions and mortality occurred across the age spectrum.

Conclusions

Both virus and host factors contributed to influenza burden. We identified age differences in comorbidities, presenting symptoms, and adverse clinical outcomes among those hospitalized with influenza and benefit from influenza vaccination in protecting against adverse clinical outcomes. The GIHSN provides an ongoing platform for global understanding of hospitalized influenza illness.

The Effect of Combining the COVID-19 Vaccine with the Seasonal Influenza Vaccine on Reducing COVID-19 Vaccine Rejection Among Libyans

BACKGROUND

Coronavirus disease 2019 (COVID-19) vaccine coverage remains low in Libya compared to other countries in the Eastern Mediterranean Region. This study aimed to evaluate the willingness of the general public in Libya to receive COVID-19 and seasonal influenza vaccines. Additionally, the study aimed to investigate the potential effect of combining the two vaccines to reduce COVID-19 vaccine rejection.

METHODS

An anonymous nationwide online cross-sectional survey was carried out from 1st September to 16th October 2022. Libyans aged 18 years or older were recruited using convenience and snowball sampling approaches. The participants were surveyed for sociodemographic information, health status, and vaccination attitude towards COVID-19 and seasonal influenza vaccines.

RESULTS

A total of 2484 participants formed the final study sample: 68.7% were females, 39.4% were aged 18-25 years, 50.4% were single, 32.5% had previous COVID-19 infection, and 47.2% experienced COVID-19 death among relatives. Three-fourths of the respondents showed COVID-19 vaccine rejection: 57.3% did not receive COVID-19 vaccination, 10.1% would not complete the primary vaccination series, and 7.8% refused booster doses. About 55.0% rejected seasonal influenza vaccination, while 1.9% reported influenza vaccine uptake and 21.2% were willing to get the influenza vaccine for the first time. Additionally, 18.8% had already received influenza vaccination in the last year and intended to get the vaccine this season, while 3.3% were unwilling to get influenza vaccination this year despite receiving it in the last influenza season. Age, sex, and occupation were significantly associated with COVID-19 and influenza vaccine rejection. Rejection of COVID-19 vaccination decreased if its combination with influenza vaccine as a single dose was suggested, with 28.2% of the COVID-19 vaccine rejector group accepting the combined vaccine as it would be safer (50.9%), needing fewer injections (24.0%), would be more effective (19.1%), and would be less expensive (3%). Approximately 73.0% of the COVID-19 vaccine rejector group refused this combination due to fear of side effects (48.7%), absence of published studies on this combination (29.8%), and considering this combination as useless (11.2%).

CONCLUSION

In Libya, the prevalence of COVID-19 vaccine rejection was high, while the rejection of seasonal influenza vaccination was relatively lower. If influenza and COVID-19 vaccines are administered simultaneously as a single injection, this may reduce the rejection of the COVID-19 vaccine due to better-perceived vaccine safety and efficacy besides being more convenient in terms of the number of injections and cost.

Factors Influencing the Uptake of Seasonal Influenza Vaccination among Community-Dwelling Older Adults during COVID-19: A Mixed Methods Study

Background: Despite making the influenza vaccine accessible and affordable, vaccination rates remained low among community-dwelling older adults. Therefore, this study aimed to explore the factors influencing vaccine uptake and the impact of COVID-19 on vaccine uptake among community-dwelling older adults in Singapore. Methods: A mixed methods study involving a survey and semi-structured interviews were conducted between September 2020 and July 2021. Community-dwelling older adults aged ≥ 65 years were recruited from 27 Community Nurse Posts. Data on participants’ demographics, health condition(s), vaccination status, attitudes towards influenza infections and vaccinations, willingness to pay, intention for future vaccination and source of information were collected via the survey. Semi-structured interviews were conducted to understand vaccination experiences, key enablers and barriers, and the impact of COVID-19 on vaccine uptake. All interviews were analysed using Braun and Clarke’s thematic analysis. Quantitative data were analysed using descriptive statistics, chi-square tests and multinomial logistic regressions. Results: A total of 235 participants completed the survey. Living arrangement was a statistically significant contributing factor for influenza vaccine uptake (ꭓ2= −0.139; p = 0.03). Participants who lived alone were 2.5 times more likely to be vaccinated than those living with others (OR = 2.504, 95% CI: 1.294–4.842, p = 0.006). Avoidance of getting infected (82.5%), avoidance of transmission to others (84.7%), and advice from healthcare professionals to receive vaccination (83.4%) were key enablers, while concerns about possible side effects (41.2%), the effectiveness of the vaccine (42.6%), and not having enough information (48.1%) were barriers. Twenty participants were interviewed. The findings were congruent with the survey results. Five themes were identified as follows: (1) Perceived importance of influenza vaccination, (2) Sphere of influence, (3) Healthcare schemes and medical subsidies, (4) Psychological impediments, and (5) Inconsistent emphases at various touch points. Conclusions: Greater public health efforts are needed to reach out to the larger population of older adults of different living arrangements and those concerned about the possible side effects and effectiveness of the influenza vaccine. Healthcare professionals need to provide more information to address these concerns, especially during COVID-19, to encourage vaccine uptake.

Evaluation of missed influenza vaccination opportunities in the emergency department

BACKGROUND

Seasonal influenza is associated with significant healthcare resource utilization. An estimated 490,000 hospitalizations and 34,000 deaths were attributed to influenza during the 2018-2019 season. Despite robust influenza vaccination programs in both the inpatient and outpatient setting, the emergency department (ED) represents a missed opportunity to vaccinate patients at high risk for influenza who do not have access to routine preventive care. Feasibility and implementation of ED-based influenza vaccination programs have been previously described but have stopped short of describing the predicted health resource impact. The goal of our study was to describe the potential impact of an influenza vaccination program in an urban adult emergency department population using historic patient data.

METHODS

This was a retrospective study of all encounters within a tertiary care hospital-based ED and three freestanding EDs during influenza season (defined as October 1 - April 30) over a two-years, 2018-2020. Data was obtained from the electronic medical record (EPIC®). All ED encounters during the study period were screened for inclusion using ICD 10 codes. Patients with a confirmed positive influenza test and no documented influenza vaccine for the current season were reviewed for any ED encounter at least 14 days prior to the influenza-positive encounter and during the concurrent influenza season. These ED visits were deemed a missed opportunity to provide vaccination and potentially prevent the influenza-positive encounter. Healthcare resource utilization, including subsequent ED encounters and inpatient admissions, were evaluated for patients with a missed vaccination opportunity.

RESULTS

A total of 116,140 ED encounters occurred during the study and were screened for inclusion. Of these, 2115 were influenza-positive encounters, which represented 1963 unique patients. There were 418 patients (21.3%) that had a missed opportunity to be vaccinated during an ED encounter at least 14 days prior to the influenza-positive encounter. Of those with a missed vaccination opportunity, 60 patients (14.4%) had subsequent influenza-related encounters, including 69 ED visits and 7 inpatient admissions.

CONCLUSION

Patients presenting to the ED with influenza frequently had opportunities to be vaccinated during prior ED encounters. An ED-based influenza vaccination program could potentially reduce influenza-related burden on healthcare resources by preventing future influenza-related ED encounters and hospitalizations.

Double Negative Control Inference in Test-Negative Design Studies of Vaccine Effectiveness

The test-negative design (TND) has become a standard approach to evaluate vaccine effectiveness against the risk of acquiring infectious diseases in real-world settings, such as Influenza, Rotavirus, Dengue fever, and more recently COVID-19. In a TND study, individuals who experience symptoms and seek care are recruited and tested for the infectious disease which defines cases and controls. Despite TND's potential to reduce unobserved differences in healthcare seeking behavior (HSB) between vaccinated and unvaccinated subjects, it remains subject to various potential biases. First, residual confounding bias may remain due to unobserved HSB, occupation as healthcare worker, or previous infection history. Second, because selection into the TND sample is a common consequence of infection and HSB, collider stratification bias may exist when conditioning the analysis on testing, which further induces confounding by latent HSB. In this paper, we present a novel approach to identify and estimate vaccine effectiveness in the target population by carefully leveraging a pair of negative control exposure and outcome variables to account for potential hidden bias in TND studies. We illustrate our proposed method with extensive simulation and an application to study COVID-19 vaccine effectiveness using data from the University of Michigan Health System.

Household Transmission of Influenza A Viruses in 2021-2022

IMPORTANCE

Influenza virus infections declined globally during the COVID-19 pandemic. Loss of natural immunity from lower rates of influenza infection and documented antigenic changes in circulating viruses may have resulted in increased susceptibility to influenza virus infection during the 2021-2022 influenza season.

OBJECTIVE

To compare the risk of influenza virus infection among household contacts of patients with influenza during the 2021-2022 influenza season with risk of influenza virus infection among household contacts during influenza seasons before the COVID-19 pandemic in the US.

DESIGN, SETTING, AND PARTICIPANTS

This prospective study of influenza transmission enrolled households in 2 states before the COVID-19 pandemic (2017-2020) and in 4 US states during the 2021-2022 influenza season. Primary cases were individuals with the earliest laboratory-confirmed influenza A(H3N2) virus infection in a household. Household contacts were people living with the primary cases who self-collected nasal swabs daily for influenza molecular testing and completed symptom diaries daily for 5 to 10 days after enrollment.

EXPOSURES

Household contacts living with a primary case.

MAIN OUTCOMES AND MEASURES

Relative risk of laboratory-confirmed influenza A(H3N2) virus infection in household contacts during the 2021-2022 season compared with prepandemic seasons. Risk estimates were adjusted for age, vaccination status, frequency of interaction with the primary case, and household density. Subgroup analyses by age, vaccination status, and frequency of interaction with the primary case were also conducted.

RESULTS

During the prepandemic seasons, 152 primary cases (median age, 13 years; 3.9% Black; 52.0% female) and 353 household contacts (median age, 33 years; 2.8% Black; 54.1% female) were included and during the 2021-2022 influenza season, 84 primary cases (median age, 10 years; 13.1% Black; 52.4% female) and 186 household contacts (median age, 28.5 years; 14.0% Black; 63.4% female) were included in the analysis. During the prepandemic influenza seasons, 20.1% (71/353) of household contacts were infected with influenza A(H3N2) viruses compared with 50.0% (93/186) of household contacts in 2021-2022. The adjusted relative risk of A(H3N2) virus infection in 2021-2022 was 2.31 (95% CI, 1.86-2.86) compared with prepandemic seasons.

CONCLUSIONS AND RELEVANCE

Among cohorts in 5 US states, there was a significantly increased risk of household transmission of influenza A(H3N2) in 2021-2022 compared with prepandemic seasons. Additional research is needed to understand reasons for this association.

A rapid and flexible microneutralization assay for serological assessment of influenza viruses

BACKGROUND

Serological responses from influenza vaccination or infection are typically measured by hemagglutinin inhibition (HAI) or microneutralization (MN). Both methods are limited in feasibility, standardization, and generalizability to recent strains. We developed a luciferase MN (LMN) assay that combines the advantages of the conventional MN assay with the ease of the HAI assay.

METHODS

Sera were obtained from the HIVE study, a Michigan household cohort. Reverse genetics was used to generate recombinant influenza viruses expressing the hemagglutinin and neuraminidase of test strains, all other viral proteins from an A/WSN/1933 backbone, and a NanoLuc reporter. Serum neutralization of luciferase-expressing targets was quantified as a reduction in light emission from infected cells. Neutralization titers were measured for cell- and egg-adapted versions of A/Hong Kong/4801/2014 and A/Singapore/INFIMH-16-0019/2016 and compared to HAI titers against egg-grown antigens.

RESULTS

Three hundred thirty-three sera were collected from 259 participants between May 2016 and July 2018. Sampled participants were 7-68 years of age, and >80% were vaccinated against influenza. HAI and LMN titers were correlated for A/Hong Kong/4801/2014 (ρ = 0.52, p ≤ 0.01) and A/Singapore/INFIMH-16-0019/2016 (ρ = 0.79, p ≤ 0.01). LMN titers were lower for cell strains compared to egg strains (A/Hong Kong/4801/2014 mean log2 fold change = -2.66, p ≤ 0.01 and A/Singapore/INFIMH-16-0019/2016 mean log2 fold change = -3.15, p ≤ 0.01).

CONCLUSIONS

The LMN assay was feasible using limited sample volumes and able to differentiate small antigenic differences between egg-adapted and cell-derived strains. The correspondence of these results with the commonly used HAI confirms the utility of this assay for high-throughput studies of correlates of protection and vaccine response.

Study on the influence of meteorological factors on influenza in different regions and predictions based on an LSTM algorithm

BACKGROUND

Influenza epidemics pose a threat to human health. It has been reported that meteorological factors (MFs) are associated with influenza. This study aimed to explore the similarities and differences between the influences of more comprehensive MFs on influenza in cities with different economic, geographical and climatic characteristics in Fujian Province. Then, the information was used to predict the daily number of cases of influenza in various cities based on MFs to provide bases for early warning systems and outbreak prevention.

METHOD

Distributed lag nonlinear models (DLNMs) were used to analyse the influence of MFs on influenza in different regions of Fujian Province from 2010 to 2021. Long short-term memory (LSTM) was used to train and model daily cases of influenza in 2010-2018, 2010-2019, and 2010-2020 based on meteorological daily values. Daily cases of influenza in 2019, 2020 and 2021 were predicted. The root mean squared error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE) and symmetric mean absolute percentage error (SMAPE) were used to quantify the accuracy of model predictions.

RESULTS

The cumulative effect of low and high values of air pressure (PRS), air temperature (TEM), air temperature difference (TEMD) and sunshine duration (SSD) on the risk of influenza was obvious. Low (< 979 hPa), medium (983 to 987 hPa) and high (> 112 hPa) PRS were associated with a higher risk of influenza in women, children aged 0 to 12 years, and rural populations. Low (< 9 °C) and high (> 23 °C) TEM were risk factors for influenza in four cities. Wind speed (WIN) had a more significant effect on the risk of influenza in the ≥ 60-year-old group. Low (< 40%) and high (> 80%) relative humidity (RHU) in Fuzhou and Xiamen had a significant effect on influenza. When PRS was between 1005-1015 hPa, RHU > 60%, PRE was low, TEM was between 10-20 °C, and WIN was low, the interaction between different MFs and influenza was most obvious. The RMSE, MAE, MAPE, and SMAPE evaluation indices of the predictions in 2019, 2020 and 2021 were low, and the prediction accuracy was high.

CONCLUSION

All eight MFs studied had an impact on influenza in four cities, but there were similarities and differences. The LSTM model, combined with these eight MFs, was highly accurate in predicting the daily cases of influenza. These MFs and prediction models could be incorporated into the influenza early warning and prediction system of each city and used as a reference to formulate prevention strategies for relevant departments.

Burden of medically attended influenza infection and cases averted by vaccination - United States, 2016/17 through 2018/19 influenza seasons

BACKGROUND

Epidemics of seasonal influenza vary in intensity annually, and influenza vaccine effectiveness (VE) fluctuates based in part on antigenic match to circulating viruses. We estimated the incidence of influenza and influenza cases averted by vaccination in four ambulatory care sites in the United States, during seasons when overall influenza VE ranged from 29% to 40%.

METHODS

We conducted active surveillance for influenza at ambulatory care settings at four sites within the United States Influenza Vaccine Effectiveness Network. We extrapolated the total number of influenza cases in the source populations served by these organizations based on incidence of medically attended acute respiratory illness in the source population and influenza test results in those actively tested for influenza. We estimated the number of medically attended influenza cases averted based on incidence, vaccine coverage, and VE.

RESULTS

From 2016/17 through 2018/19, incidence of ambulatory visits for laboratory-confirmed influenza ranged from 31 to 51 per 1,000 population. Incidence was highest in children aged 9-17 years (range, 56 to 81 per 1,000) and lowest in adults aged 18-49 years (range, 23-32 per 1,000). Medically attended cases averted by vaccination ranged from a high of 46.6 (95 % CI, 12.1- 91.9) per 1,000 vaccinees in children aged 6 months to 8 years, to a low of 6.9 (95 % CI, -5.1- 27.3) per 1,000 vaccinees in adults aged ≥ 65 years.

DISCUSSION

Even in seasons with low vaccine effectiveness for a particular virus subtype, influenza vaccines can still lead to clinically meaningful reductions in ambulatory care visits for influenza.

The effectiveness of influenza vaccine among elderly Chinese: A regression discontinuity design based on Yinzhou regional health information platform

In China, a free influenza vaccination policy is being implemented among individuals aged 70 years and over in Zhejiang province during the COVID-19 pandemic. The objective was to assess the effectiveness of influenza vaccine in reducing hospitalization and mortality in the elderly. We used data from the Regional Health Information Platform in Yinzhou located in Zhejiang province and applied a regression discontinuity design to estimate the intention-to-treat effect on admission and mortality rates by month of age in the population who was near the age of 70 years threshold. At age 70 years, the influenza vaccination rate increased by 29.1% (95% CI, 28.2% to 29.9%) compared to those under 70 in the study population. When turning age 70 years, the potential effectiveness of receiving influenza vaccine was 8.2% (95% CI, -36.8% to 51.3%) for total hospitalization and the evaluation of vaccine effectiveness was 13.1% (95% CI, -34.2 to 61.8) for the all-cause mortality. An increase in the influenza vaccination rate was associated with a weak decline in most outcomes, but no significance was found for all outcomes. Influenza vaccination had a limited effect on hospital admission and mortality for the free influenza vaccination program that can be related to the low vaccination rate among the Chinese elderly. Supplementation strategies and future studies may be needed to expand immunization coverage and validate this finding, and further provide a reference for other cities to promote the free influenza vaccination policy in China, especially under circumstances of the COVID-19 pandemic.

Recommendations for Prevention and Control of Influenza in Children, 2022-2023

This technical report accompanies the recommendations of the American Academy of Pediatrics for the routine use of influenza vaccine and antiviral medications in the prevention and treatment of influenza in children during the 2022 to 2023 season. The American Academy of Pediatrics recommends annual influenza vaccination of all children without medical contraindications starting at 6 months of age. Influenza vaccination is an important strategy for protecting children and the broader community as well as reducing the overall burden of respiratory illnesses when other viruses, including severe acute respiratory syndrome-coronavirus 2, are cocirculating. This technical report summarizes recent influenza seasons, morbidity and mortality in children, vaccine effectiveness, and vaccination coverage, and provides detailed guidance on storage, administration, and implementation. The report also provides a brief background on inactivated and live attenuated influenza vaccine recommendations, vaccination during pregnancy and breastfeeding, diagnostic testing, and antiviral medications for treatment and chemoprophylaxis. Updated information is provided about the 2021 to 2022 influenza season, influenza immunization rates, the effectiveness of influenza vaccination on hospitalization and mortality, available vaccines, guidance for patients with history of severe allergic reactions to prior influenza vaccinations, and strategies to promote vaccine uptake.

Impact of Routine Childhood Immunization in Reducing Vaccine-Preventable Diseases in the United States

BACKGROUND AND OBJECTIVES

Current routine immunizations for children aged ≤10 years in the United States in 2019 cover 14 vaccine-preventable diseases. We characterize the public-health impact of vaccination by providing updated estimates of disease incidence with and without universally recommended pediatric vaccines.

METHODS

Prevaccine disease incidence was obtained from published data or calculated using annual case estimates from the prevaccine period and United States population estimates during the same period. Vaccine-era incidence was calculated as the average incidence over the most recent 5 years of available surveillance data or obtained from published estimates (if surveillance data were not available). We adjusted for underreporting and calculated the percent reduction in overall and age-specific incidence for each disease. We multiplied prevaccine and vaccine-era incidence rates by 2019 United States population estimates to calculate annual number of cases averted by vaccination.

RESULTS

Routine immunization reduced the incidence of all targeted diseases, leading to reductions in incidence ranging from 17% (influenza) to 100% (diphtheria, Haemophilus influenzae type b, measles, mumps, polio, and rubella). For the 2019 United States population of 328 million people, these reductions equate to >24 million cases of vaccine-preventable disease averted. Vaccine-era disease incidence estimates remained highest for influenza (13 412 per 100 000) and Streptococcus pneumoniae-related acute otitis media (2756 per 100 000).

CONCLUSIONS

Routine childhood immunization in the United States continues to yield considerable sustained reductions in incidence across all targeted diseases. Efforts to maintain and improve vaccination coverage are necessary to continue experiencing low incidence levels of vaccine-preventable diseases.

Modelling the Economic Impact of lnfluenza Vaccine Programs with the Cell-Based Quadrivalent Influenza Vaccine and Adjuvanted Trivalent Influenza Vaccine in Canada

In Canada, approximately 12,000 people annually are hospitalized with influenza. While vaccination is the most effective method for reducing the burden of seasonal influenza, the propagation of vaccine virus strains in eggs can result in egg adaption, resulting in reduced antigenic similarity to circulating strains and thus lower vaccine effectiveness (VE). Cell-based propagation methods avoid these alterations and therefore may be more effective than egg-propagation vaccines. We evaluated three different scenarios: (1) egg-based quadrivalent influenza vaccine (QIVe) for individuals <65 years and adjuvanted trivalent influenza vaccine (aTIV) for ≥65 years; (2) QIVe (<65 years) and high-dose QIV (HD −; QIV; ≥65 years); and (3) cell-based derived QIV (QIVc; <65 years) and aTIV (≥65 years) compared with a baseline scenario of QIVe for all age groups. Modelling was performed using a dynamic age-structured SEIR model, which assessed each strain individually using data from the 2012−2019 seasons. Probabilistic sensitivity analysis assessed the robustness of the results with respect to variation in absolute VE, relative VE, number of egg-adapted seasons, and economic parameters. QIVe + aTIV was cost-saving compared with the baseline scenario (QIVe for all), and QIVe + HD − QIV was not cost-effective in the majority of simulations, reflecting the high acquisition cost of HD − QIV. Overall, while the incremental benefits may vary by influenza season, QIVc + aTIV resulted in the greatest reductions in cases, hospitalizations, and mortality, and was cost-effective (ICER < CAD 50,000) in all simulations.

Multiplex Detection of Antibody Landscapes to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)/Influenza/Common Human Coronaviruses Following Vaccination or Infection With SARS-CoV-2 and Influenza

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza viruses continue to co-circulate, representing 2 major public health threats from respiratory infections with similar clinical presentations. SARS-CoV-2 and influenza vaccines can also now be co-administered. However, data on antibody responses to SARS-CoV-2 and influenza coinfection and vaccine co-administration remain limited.

METHODS

We developed a 41-plex antibody immunity assay that can simultaneously characterize antibody landscapes to SARS-CoV-2/influenza/common human coronaviruses. We analyzed sera from 840 individuals (11-93 years), including sera from reverse transcription-polymerase chain reaction (RT-PCR)-confirmed SARS-CoV-2-positive (n = 218) and -negative (n = 120) cases, paired sera from SARS-CoV-2 vaccination (n = 29) and infection (n = 11), and paired sera from influenza vaccination (n = 56) and RT-PCR-confirmed influenza infection (n = 158) cases. Last, we analyzed sera collected from 377 individuals who exhibited acute respiratory illness (ARI) in 2020.

RESULTS

This 41-plex assay has high sensitivity and specificity in detecting SARS-CoV-2 infections. It differentiated SARS-CoV-2 vaccination (antibody responses only to spike protein) from infection (antibody responses to both spike and nucleoprotein). No cross-reactive antibodies were induced to SARS-CoV-2 from influenza vaccination and infection, and vice versa, suggesting no interaction between SARS-CoV-2 and influenza antibody responses. However, cross-reactive antibodies were detected between spike proteins of SARS-CoV-2 and common human coronaviruses that were removed by serum adsorption. Among 377 individuals who exhibited ARI in 2020, 129 were influenza positive; none had serological evidence of SARS-CoV-2/influenza coinfections.

CONCLUSIONS

Multiplex detection of antibody landscapes can provide in-depth analysis of the antibody protective immunity to SARS-CoV-2 in the context of other respiratory viruses, including influenza.

Days of Flooding Associated with Increased Risk of Influenza

Influenza typically causes mild infection but can lead to severe outcomes for those with compromised lung health. Flooding, a seasonal problem in Iowa, can expose many Iowans to molds and allergens shown to alter lung inflammation, leading to asthma attacks and decreased viral clearance. Based on this, the hypothesis for this research was that there would be geographically specific positive associations in locations with flooding with influenza diagnosis. An ecological study was performed using influenza diagnoses and positive influenza polymerase chain reaction tests from a de-identified large private insurance database and Iowa State Hygienic Lab. After adjustment for multiple confounding factors, Poisson regression analysis resulted in a consistent 1% associated increase in influenza diagnoses per day above flood stage (95% confidence interval: 1.00–1.04). This relationship remained after removal of the 2009–2010 influenza pandemic year. There was no associated risk between flooding and influenza-like illness as a nonspecific diagnosis. Associated risks between flooding and increased influenza diagnoses were geographically specific, with the greatest risk in the most densely populated areas. This study indicates that populations who live, work, or volunteer in flooded environments should consider preventative measures to avoid environmental exposures to mitigate illness from influenza in the following year.

Economic Burden of COVID-19: A Systematic Review

Objective

To review and qualitatively synthesize the evidence related to the economic burden of COVID-19, including healthcare resource utilization and costs.

Methods

A systematic review of studies that assessed the economic burden [eg, direct costs, productivity, macroeconomic impact due to non-pharmaceutical interventions (NPIs) and equity] of COVID-19 was conducted by searches in EMBASE, MEDLINE, MEDLINE-IN-PROCESS, and The Cochrane Library, as well as manual searches of unpublished research for the period between January 2020 to February 2021. Single reviewer data extraction was confirmed independently by a second reviewer.

Results

The screening process resulted in a total of 27 studies: 25 individual publications, and 2 systematic literature reviews, of narrower scopes, that fulfilled the inclusion criteria. The patients diagnosed with more severe COVID-19 were associated with higher costs. The main drivers for higher costs were consistent across countries and included ICU admission, in-hospital resource use such as mechanical ventilation, which lead to increase costs of $2082.65 ± 345.04 to $2990.76 ± 545.98. The most frequently reported indirect costs were due to productivity losses. On average, older COVID-19 patients incurred higher costs when compared to younger age groups. An estimation of a 20% COVID-19 infection rate based on a Monte Carlo simulation in the United States led to a total direct medical cost of $163.4 billion over the course of the pandemic.

Conclusion

The COVID-19 pandemic has generated a considerable economic burden on patients and the general population. Preventative measures such as NPIs only have partial success in lowering the economic costs of the pandemic. Implementing additional preventative measures such as large-scale vaccination is vital in reducing direct and indirect medical costs, decreased productivity, and GDP losses.

Reduced COVID-19 hospitalizations among New York City residents following age-based SARS-CoV-2 vaccine eligibility: Evidence from a regression discontinuity design

BACKGROUND

In clinical trials, several SARS-CoV-2 vaccines were shown to reduce risk of severe COVID-19 illness. Local, population-level, real-world evidence of vaccine effectiveness is accumulating. We assessed vaccine effectiveness for community-dwelling New York City (NYC) residents using a quasi-experimental, regression discontinuity design, leveraging a period (January 12-March 9, 2021) when ≥ 65-year-olds were vaccine-eligible but younger persons, excluding essential workers, were not.

METHODS

We constructed segmented, negative binomial regression models of age-specific COVID-19 hospitalization rates among 45-84-year-old NYC residents during a post-vaccination program implementation period (February 21-April 17, 2021), with a discontinuity at age 65 years. The relationship between age and hospitalization rates in an unvaccinated population was incorporated using a pre-implementation period (December 20, 2020-February 13, 2021). We calculated the rate ratio (RR) and 95% confidence interval (CI) for the interaction between implementation period (pre or post) and age-based eligibility (45-64 or 65-84 years). Analyses were stratified by race/ethnicity and borough of residence. Similar analyses were conducted for COVID-19 deaths.

RESULTS

Hospitalization rates among 65-84-year-olds decreased from pre- to post-implementation periods (RR 0.85, 95% CI: 0.74-0.97), controlling for trends among 45-64-year-olds. Accordingly, an estimated 721 (95% CI: 126-1,241) hospitalizations were averted. Residents just above the eligibility threshold (65-66-year-olds) had lower hospitalization rates than those below (63-64-year-olds). Racial/ethnic groups and boroughs with higher vaccine coverage generally experienced greater reductions in RR point estimates. Uncertainty was greater for the decrease in COVID-19 death rates (RR 0.85, 95% CI: 0.66-1.10).

CONCLUSION

The vaccination program in NYC reduced COVID-19 hospitalizations among the initially age-eligible ≥ 65-year-old population by approximately 15% in the first eight weeks. The real-world evidence of vaccine effectiveness makes it more imperative to improve vaccine access and uptake to reduce inequities in COVID-19 outcomes.

Bivalent H1 and H3 COBRA Recombinant Hemagglutinin Vaccines Elicit Seroprotective Antibodies against H1N1 and H3N2 Influenza Viruses from 2009 to 2019

Commercial influenza virus vaccines often elicit strain-specific immune responses and have difficulties preventing illness caused by antigenically drifted viral variants. In the last 20 years, the H3N2 component of the annual vaccine has been updated nearly twice as often as the H1N1 component, and in 2019, a mismatch between the wild-type (WT) H3N2 vaccine strain and circulating H3N2 influenza strains led to a vaccine efficacy of ∼9%. Modern methods of developing computationally optimized broadly reactive antigens (COBRAs) for H3N2 influenza viruses utilize current viral surveillance information to design more broadly reactive vaccine antigens. Here, 7 new recombinant hemagglutinin (rHA) H3 COBRA hemagglutinin (HA) antigens were evaluated in mice. Subsequently, two candidates, J4 and NG2, were selected for further testing in influenza-preimmune animals based on their ability to elicit broadly reactive antibodies against antigenically drifted H3N2 viral isolates. In the preimmune model, monovalent formulations of J4 and NG2 elicited broadly reactive antibodies against recently circulating H3N2 influenza viruses from 2019. Bivalent mixtures of COBRA H1 and H3 rHA, Y2 + J4, and Y2 + NG2 outperformed multiple WT H1+H3 bivalent rHA mixtures by eliciting seroprotective antibodies against H1N1 and H3N2 isolates from 2009 to 2019. Overall, the newly generated COBRA HA antigens, namely, Y2, J4, and NG2, had the ability to induce broadly reactive antibodies in influenza-naive and preimmune animals in both monovalent and bivalent formulations, and these antigens outperformed H1 and H3 WT rHA vaccine antigens by eliciting seroprotective antibodies against panels of antigenically drifted historical H1N1 and H3N2 vaccine strains from 2009 to 2019. IMPORTANCE Standard-of-care influenza virus vaccines are composed of a mixture of antigens from different influenza viral subtypes. For the first time, lead COBRA H1 and H3 HA antigens, formulated as a bivalent vaccine, have been investigated in animals with preexisting immunity to influenza viruses. The cocktail of COBRA HA antigens elicited more broadly reactive anti-HA antibodies than those elicited by a comparator bivalent wild-type HA vaccine against H1 and H3 influenza viruses isolated between 2009 and 2019.

Characteristics and Outcomes of Hospitalized Pregnant Women With Influenza, 2010 to 2019 : A Repeated Cross-Sectional Study

BACKGROUND

Pregnant women may be at increased risk for severe influenza-associated outcomes.

OBJECTIVE

To describe characteristics and outcomes of hospitalized pregnant women with influenza.

DESIGN

Repeated cross-sectional study.

SETTING

The population-based U.S. Influenza Hospitalization Surveillance Network during the 2010-2011 through 2018-2019 influenza seasons.

PATIENTS

Pregnant women (aged 15 to 44 years) hospitalized with laboratory-confirmed influenza identified through provider-initiated or facility-based testing practices.

MEASUREMENTS

Clinical characteristics, interventions, and in-hospital maternal and fetal outcomes were obtained through medical chart abstraction. Multivariable logistic regression was used to evaluate the association between influenza A subtype and severe maternal influenza-associated outcomes, including intensive care unit (ICU) admission, mechanical ventilation, extracorporeal membrane oxygenation, or in-hospital death.

RESULTS

Of 9652 women aged 15 to 44 years and hospitalized with influenza, 2690 (27.9%) were pregnant. Among the 2690 pregnant women, the median age was 28 years, 62% were in their third trimester, and 42% had at least 1 underlying condition. Overall, 32% were vaccinated against influenza and 88% received antiviral treatment. Five percent required ICU admission, 2% required mechanical ventilation, and 0.3% (n = 8) died. Pregnant women with influenza A H1N1 were more likely to have severe outcomes than those with influenza A H3N2 (adjusted risk ratio, 1.9 [95% CI, 1.3 to 2.8]). Most women (71%) were still pregnant at hospital discharge. Among 754 women who were no longer pregnant at discharge, 96% had a pregnancy resulting in live birth, and 3% experienced fetal loss.

LIMITATION

Maternal and fetal outcomes that occurred after hospital discharge were not captured.

CONCLUSION

Over 9 influenza seasons, one third of reproductive-aged women hospitalized with influenza were pregnant. Influenza A H1N1 was associated with more severe maternal outcomes. Pregnant women remain a high-priority target group for vaccination.

PRIMARY FUNDING SOURCE

Centers for Disease Control and Prevention.

Subtype H3N2 Influenza A Viruses: An Unmet Challenge in the Western Pacific

Subtype H3N2 influenza A viruses (A(H3N2)) have been the dominant strain in some countries in the Western Pacific region since the 2009 influenza A(H1N1) pandemic. Vaccination is the most effective way to prevent influenza; however, low vaccine effectiveness has been reported in some influenza seasons, especially for A(H3N2). Antigenic mismatch introduced by egg-adaptation during vaccine production between the vaccine and circulating viral stains is one of the reasons for low vaccine effectiveness. Here we review the extent of this phenomenon, the underlying molecular mechanisms and discuss recent strategies to ameliorate this, including new vaccine platforms that may provide better protection and should be considered to reduce the impact of A(H3N2) in the Western Pacific region.

Operationalizing influenza vaccination in an urban safety-net emergency department

INTRODUCTION

Influenza vaccination is a recommended tool in preventing influenza-related illnesses, medical visits, and hospitalizations. With many patients remaining unvaccinated each year, the Emergency Department (ED) represents a unique opportunity to provide vaccinations to patient not yet vaccinated. However, busy urban safety-net EDs maybe challenged to safely execute such a vaccination program. The aim of this quality improvement project was to assess influenza vaccination feasibility in the ED and improve influenza vaccination rates in our community.

METHODS

The quality improvement work-group, comprised of ED physicians, nurses, and pharmacists, designed and implemented an influenza vaccination protocol that aligned with the ED workflow. The outcome measure was the total number of patients vaccinated per month and per influenza season. Process measures included the type of influenza vaccine administered and type of care area within ED. Balancing measures were also included.

RESULTS

Following the initiative, a total of 337 patients received influenza vaccinations in the ED between September 1, 2018 and December 31, 2020 compared to none during the previous influenza season. With each influenza season, the number of vaccinated patients increased from 61 to 134 and 142, respectively. The average age of the patients was 48.23 ± 15.29, 52.89 ± 15.91, and 44.92 ± 18.97 years old. Most patients received the vaccination while roomed in the high acuity section of the adult ED. No adverse effects or automated dispensing cabinet stockouts were observed.

CONCLUSION

Our structured program indicates that influenza vaccine administration to eligible patients is feasible in a busy urban safety-net ED. Piloting new and further developing existing ED-based influenza vaccination programs have the potential to significantly benefit public health.

Human Influenza Epidemiology

Influenza virus infections are common in people of all ages. Epidemics occur in the winter months in temperate locations and at varying times of the year in subtropical and tropical locations. Most influenza virus infections cause mild and self-limiting disease, and around one-half of all infections occur with a fever. Only a small minority of infections lead to serious disease requiring hospitalization. During epidemics, the rates of influenza virus infections are typically highest in school-age children. The clinical severity of infections tends to increase at the extremes of age and with the presence of underlying medical conditions, and impact of epidemics is greatest in these groups. Vaccination is the most effective measure to prevent infections, and in recent years influenza vaccines have become the most frequently used vaccines in the world. Nonpharmaceutical public health measures can also be effective in reducing transmission, allowing suppression or mitigation of influenza epidemics and pandemics.

Recommendations for Prevention and Control of Influenza in Children, 2021-2022

This technical report accompanies the recommendations of the American Academy of Pediatrics for the routine use of the influenza vaccine and antiviral medications in the prevention and treatment of influenza in children during the 2021-2022 season. Influenza vaccination is an important intervention to protect vulnerable populations and reduce the burden of respiratory illnesses during circulation of severe acute respiratory syndrome coronavirus 2, which is expected to continue during this influenza season. In this technical report, we summarize recent influenza seasons, morbidity and mortality in children, vaccine effectiveness, vaccination coverage, and detailed guidance on storage, administration, and implementation. We also provide background on inactivated and live attenuated influenza vaccine recommendations, vaccination during pregnancy and breastfeeding, diagnostic testing, and antiviral medications for treatment and chemoprophylaxis.

Evaluation of Next-Generation H3 Influenza Vaccines in Ferrets Pre-Immune to Historical H3N2 Viruses

Each person has a unique immune history to past influenza virus infections. Exposure to influenza viruses early in life establishes memory B cell populations that influence future immune responses to influenza vaccination. Current influenza vaccines elicit antibodies that are typically strain specific and do not offer broad protection against antigenically drifted influenza strains in all age groups of people. This is particularly true for vaccine antigens of the A(H3N2) influenza virus subtype, where continual antigenic drift necessitates frequent vaccine reformulation. Broadly-reactive influenza virus vaccine antigens offer a solution to combat antigenic drift, but they also need to be equally effective in all populations, regardless of prior influenza virus exposure history. This study examined the role that pre-existing immunity plays on influenza virus vaccination. Ferrets were infected with historical A(H3N2) influenza viruses isolated from either the 1970’s, 1980’s, or 1990’s and then vaccinated with computationally optimized broadly reactive antigens (COBRA) or wild-type (WT) influenza virus like particles (VLPs) expressing hemagglutinin (HA) vaccine antigens to examine the expansion of immune breadth. Vaccines with the H3 COBRA HA antigens had more cross-reactive antibodies following a single vaccination in all three pre-immune regimens than vaccines with WT H3 HA antigens against historical, contemporary, and future drifted A(H3N2) influenza viruses. The H3 COBRA HA vaccines also induced antibodies capable of neutralizing live virus infections against modern drifted A(H3N2) strains at higher titers than the WT H3 HA vaccine comparators.

Influenza Vaccine Effectiveness Against Hospitalization in the United States, 2019-2020

BACKGROUND

Influenza causes significant morbidity and mortality and stresses hospital resources during periods of increased circulation. We evaluated the effectiveness of the 2019-2020 influenza vaccine against influenza-associated hospitalization in the United States.

METHODS

We included adults hospitalized with acute respiratory illness at 14 hospitals and tested for influenza viruses by reserve-transcription polymerase chain reaction. Vaccine effectiveness (VE) was estimated by comparing the odds of current-season influenza vaccination in test-positive influenza cases vs test-negative controls, adjusting for confounders. VE was stratified by age and major circulating influenza types along with A(H1N1)pdm09 genetic subgroups.

RESULTS

A total of 3116 participants were included, including 18% (n = 553) influenza-positive cases. Median age was 63 years. Sixty-seven percent (n = 2079) received vaccination. Overall adjusted VE against influenza viruses was 41% (95% confidence interval [CI], 27%-52%). VE against A(H1N1)pdm09 viruses was 40% (95% CI, 24%-53%) and 33% against B viruses (95% CI, 0-56%). Of the 2 major A(H1N1)pdm09 subgroups (representing 90% of sequenced H1N1 viruses), VE against one group (5A + 187A,189E) was 59% (95% CI, 34%-75%) whereas no VE was observed against the other group (5A + 156K) (-1% [95% CI, -61% to 37%]).

CONCLUSIONS

In a primarily older population, influenza vaccination was associated with a 41% reduction in risk of hospitalized influenza illness.

Impaired memory B-cell recall responses in the elderly following recurrent influenza vaccination

Influenza is a highly contagious viral respiratory disease that affects million of people worldwide each year. Annual vaccination is recommended by the World Health Organization with the goal of reducing influenza severity and limiting transmission through elicitation of antibodies targeting the hemagglutinin (HA) glycoprotein. The antibody response elicited by current seasonal influenza virus vaccines is predominantly strain-specific, but pre-existing influenza virus immunity can greatly impact the serological antibody response to vaccination. However, it remains unclear how B cell memory is shaped by recurrent annual vaccination over the course of multiple seasons, especially in high-risk elderly populations. Here, we systematically profiled the B cell response in young adult (18-34 year old) and elderly (65+ year old) vaccine recipients that received annual split inactivated influenza virus vaccination for 3 consecutive seasons. Specifically, the antibody serological and memory B-cell compartments were profiled for reactivity against current and historical influenza A virus strains. Moreover, multiparametric analysis and antibody landscape profiling revealed a transient increase in strain-specific antibodies in the elderly, but with an impaired recall response of pre-existing memory B-cells, plasmablast (PB) differentiation and long-lasting serological changes. This study thoroughly profiles and compares the immune response to recurrent influenza virus vaccination in young and elderly participants unveiling the pitfalls of current influenza virus vaccines in high-risk populations.

Gap between willingness and behavior in the vaccination against influenza, pneumonia, and herpes zoster among Chinese aged 50-69 years

BACKGROUND

Vaccination uptake of influenza vaccine, 23-valent pneumococcal polysaccharide vaccine (PPV23) and herpes zoster vaccine (HZV) remains low among Chinese aged 50-69 years, posing a significant public health concern. This study's aim is to determine the gap between the vaccination willingness and uptake.

RESEARCH DESIGN AND METHODS

We conducted a face-to-face questionnaire survey among the persons aged 50-69 years in 13 communities in Shanghai in November-December 2020 to measure vaccination willingness, vaccine price sensitivity, vaccination service accessibility, and vaccine hesitancy. Then, we explored the factors influencing the vaccination uptake using a structural equation model.

RESULTS

Vaccination willingness was 70.5% (self-paid influenza vaccine), 79.4% (free PPV23 for local persons), 54.7% (self-paid PPV23 for non-local persons), and 16.6% (self-paid HZV); however, actual uptake was 10.8% (influenza vaccine), 11.0% (free PPV23), and 2.5% (self-paid PPV23). Moreover, vaccination uptake was strongly influenced by service accessibility (β = 0.941), followed by willingness (β = 0.201), price sensitivity (β = -0.188), and medical history of vaccine-preventable diseases and underlying diseases (β = 0.176). Related knowledge (β = 0.017) and vaccine hesitancy (β = -0.003) affected the uptake indirectly.

CONCLUSION

There is a gap between the vaccination willingness and uptake in Chinese elderly. It warrants specific measures, including increasing service accessibility and reforming payment mechanism, to bridge the gap.

Asymptomatic and Symptomatic COVID-19 Infections Among Health Care Personnel Before and After Vaccination

This cohort study investigates asymptomatic and symptomatic COVID-19 case rates before and after the initial vaccine rollout among health care personnel in Orange County, California.

Development of a Novel Live Attenuated Influenza A Virus Vaccine Encoding the IgA-Inducing Protein

Live attenuated influenza virus (LAIV) vaccines elicit a combination of systemic and mucosal immunity by mimicking a natural infection. To further enhance protective mucosal responses, we incorporated the gene encoding the IgA-inducing protein (IGIP) into the LAIV genomes of the cold-adapted A/Leningrad/134/17/57 (H2N2) strain (caLen) and the experimental attenuated backbone A/turkey/Ohio/313053/04 (H3N2) (OH/04att). Incorporation of IGIP into the caLen background led to a virus that grew poorly in prototypical substrates. In contrast, IGIP in the OH/04att background (IGIP-H1att) virus grew to titers comparable to the isogenic backbone H1att (H1N1) without IGIP. IGIP-H1att- and H1caLen-vaccinated mice were protected against lethal challenge with a homologous virus. The IGIP-H1att vaccine generated robust serum HAI responses in naïve mice against the homologous virus, equal or better than those obtained with the H1caLen vaccine. Analyses of IgG and IgA responses using a protein microarray revealed qualitative differences in humoral and mucosal responses between vaccine groups. Overall, serum and bronchoalveolar lavage samples from the IGIP-H1att group showed trends towards increased stimulation of IgG and IgA responses compared to H1caLen samples. In summary, the introduction of genes encoding immunomodulatory functions into a candidate LAIV can serve as natural adjuvants to improve overall vaccine safety and efficacy.

Influenza Virus and SARS-CoV-2 Vaccines

Seasonal influenza and the current COVID-19 pandemic represent looming global health challenges. Efficacious and safe vaccines remain the frontline tools for mitigating both influenza virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced diseases. This review will discuss the existing strategies for influenza vaccines and how these strategies have informed SARS-CoV-2 vaccines. It will also discuss new vaccine platforms and potential challenges for both viruses.

Co-Created Messaging for Influenza Vaccination in a High-Risk Hispanic Community Provides Groundwork for COVID-19 Vaccine

Purpose: Influenza/pneumonia is the eighth leading cause of death in the United States. The 2020-2021 influenza season is predicted to be further impacted by COVID-19 infections. Historical data reflect disproportionate morbidity and mortality rates in the Hispanic population for influenza and COVID-19. Influenza vaccination rates remain low in the Hispanic community. We aim to improve vaccination through a community-led event, partnering with the Cristo Rey School Dallas, located in a zip code with a higher age-adjusted influenza/pneumonia mortality rate. A survey was administered to adults attending the Influenza vaccine event to understand attitudes and perceptions about influenza, vaccination, and effective messaging strategies for the campaign. Methods: Messaging was cocreated with student health ambassadors to promote immunization and delivered through trusted sources. The health department administered vaccines to individuals >age 3 at no cost. Adults were asked to complete a 19-question survey postvaccination offered in both English and Spanish. Results: Two hundred and forty-one of 394 (61.2%) participants completed the survey. Ninety-eight percent identified as Hispanic/Latino, and the majority of surveys were administered in Spanish. Among Spanish language participants, the church bulletins (57.3%) and Spanish language radio (30.5%) were reported to be most effective modes of messaging versus word of mouth (32.9%) and social media (26.3%) for English-speaking participants. Sixteen percent of participants surveyed had never received an influenza vaccine before this event. Conclusion: Cocreated messaging delivered by trusted sources in the Hispanic community led to a successful Influenza vaccine drive with the Dallas County health department.

Selecting and Using the Appropriate Influenza Vaccine for Each Individual

Despite seasonal influenza vaccines having been routinely used for many decades, influenza A virus continues to pose a global threat to humans, causing high morbidity and mortality each year. The effectiveness of the vaccine is largely dependent on how well matched the vaccine strains are with the circulating influenza virus strains. Furthermore, low vaccine efficacy in naïve populations such as young children, or in the elderly, who possess weakened immune systems, indicates that influenza vaccines need to be more personalized to provide broader community protection. Advances in both vaccine technologies and our understanding of influenza virus infection and immunity have led to the design of a variety of alternate vaccine strategies to extend population protection against influenza, some of which are now in use. In this review, we summarize the progress in the field of influenza vaccines, including the advantages and disadvantages of different strategies, and discuss future prospects. We also highlight some of the challenges to be faced in the ongoing effort to control influenza through vaccination.

Influenza Vaccine Effectiveness for Prevention of Severe Influenza-Associated Illness among Adults in the United States, 2019-2020: A test-negative study

BACKGROUND

Influenza vaccine effectiveness (VE) against a spectrum of severe disease, including critical illness and death, remains poorly characterized.

METHODS

We conducted a test-negative study in an intensive care unit (ICU) network at 10 United States hospitals to evaluate VE for preventing influenza-associated severe acute respiratory infection (SARI) during the 2019-2020 season, which was characterized by circulation of drifted A/H1N1 and B-lineage viruses. Cases were adults hospitalized in the ICU and a targeted number outside the ICU (to capture a spectrum of severity) with laboratory-confirmed influenza-associated SARI. Test-negative controls were frequency-matched based on hospital, timing of admission, and care location (ICU vs non-ICU). Estimates were adjusted for age, comorbidities, and other confounders.

RESULTS

Among 638 patients, the median (interquartile) age was 57 (44-68) years; 286 (44.8%) patients were treated in the ICU and 42 (6.6%) died during hospitalization. Forty-five percent of cases and 61% of controls were vaccinated, which resulted in an overall VE of 32% (95% CI: 2 to 53%), including 28% (-9% to 52%) against influenza A, and 52% (13% to 74%) against influenza B. VE was higher in adults 18-49 years old (62%; 95% CI: 27% to 81%) than those 50-64 years old (20%, -48% to 57%) and ≥65 years old (-3%; 95% CI: -97% to 46%) (p=0.0789 for interaction). VE was significantly higher against influenza-associated death (80%, 95% CI: 4% to 96%) than non-fatal influenza illness.

CONCLUSIONS

During a season with drifted viruses, vaccination reduced severe influenza-associated illness among adults by 32%. VE was high among young adults.