Reduced effectiveness of repeat influenza vaccination: distinguishing among within-season waning, recent clinical infection, and subclinical infection

Studies have reported that prior-season influenza vaccination is associated with higher risk of clinical influenza infection among vaccinees. This effect might arise from incomplete consideration of within-season waning and recent infection. Using data from the US Flu Vaccine Effectiveness (VE) Network (2011-2012 to 2018-2019 seasons), we found that repeat vaccinees were vaccinated earlier in a season by one week. After accounting for waning VE, repeat vaccinees were still more likely to test positive for A(H3N2) (OR=1.11, 95%CI:1.02-1.21) but not for influenza B or A(H1N1). We found that clinical infection influenced individuals' decision to vaccinate in the following season while protecting against clinical infection of the same (sub)type. However, adjusting for recent clinical infections did not strongly influence the estimated effect of prior-season vaccination. In contrast, we found that adjusting for subclinical infection could theoretically attenuate this effect. Additional investigation is needed to determine the impact of subclinical infections on VE.

Effectiveness of Covid-19 Vaccines in Ambulatory and Inpatient Care Settings

BACKGROUND

There are limited data on the effectiveness of the vaccines against symptomatic coronavirus disease 2019 (Covid-19) currently authorized in the United States with respect to hospitalization, admission to an intensive care unit (ICU), or ambulatory care in an emergency department or urgent care clinic.

METHODS

We conducted a study involving adults (≥50 years of age) with Covid-19-like illness who underwent molecular testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We assessed 41,552 admissions to 187 hospitals and 21,522 visits to 221 emergency departments or urgent care clinics during the period from January 1 through June 22, 2021, in multiple states. The patients' vaccination status was documented in electronic health records and immunization registries. We used a test-negative design to estimate vaccine effectiveness by comparing the odds of a positive test for SARS-CoV-2 infection among vaccinated patients with those among unvaccinated patients. Vaccine effectiveness was adjusted with weights based on propensity-for-vaccination scores and according to age, geographic region, calendar time (days from January 1, 2021, to the index date for each medical visit), and local virus circulation.

RESULTS

The effectiveness of full messenger RNA (mRNA) vaccination (≥14 days after the second dose) was 89% (95% confidence interval [CI], 87 to 91) against laboratory-confirmed SARS-CoV-2 infection leading to hospitalization, 90% (95% CI, 86 to 93) against infection leading to an ICU admission, and 91% (95% CI, 89 to 93) against infection leading to an emergency department or urgent care clinic visit. The effectiveness of full vaccination with respect to a Covid-19-associated hospitalization or emergency department or urgent care clinic visit was similar with the BNT162b2 and mRNA-1273 vaccines and ranged from 81% to 95% among adults 85 years of age or older, persons with chronic medical conditions, and Black or Hispanic adults. The effectiveness of the Ad26.COV2.S vaccine was 68% (95% CI, 50 to 79) against laboratory-confirmed SARS-CoV-2 infection leading to hospitalization and 73% (95% CI, 59 to 82) against infection leading to an emergency department or urgent care clinic visit.

CONCLUSIONS

Covid-19 vaccines in the United States were highly effective against SARS-CoV-2 infection requiring hospitalization, ICU admission, or an emergency department or urgent care clinic visit. This vaccine effectiveness extended to populations that are disproportionately affected by SARS-CoV-2 infection. (Funded by the Centers for Disease Control and Prevention.).

Modeling the impacts of clinical influenza testing on influenza vaccine effectiveness estimates

BACKGROUND

Test-negative design studies for evaluating influenza vaccine effectiveness (VE) enroll patients with acute respiratory infection. Enrollment typically occurs before influenza status is determined, resulting in over-enrollment of influenza-negative patients. With availability of rapid and accurate molecular clinical testing, influenza status could be ascertained prior to enrollment, thus improving study efficiency. We estimate potential biases in VE when using clinical testing.

METHODS

We simulate data assuming 60% vaccinated, 25% of those vaccinated are influenza positive, and VE of 50%. We show the effect on VE in five scenarios.

RESULTS

VE is affected only when clinical testing preferentially targets patients based on both vaccination and influenza status. VE is overestimated by 10% if non-testing occurs in 39% of vaccinated influenza-positive patients and 24% of others; and if non-testing occurs in 8% of unvaccinated influenza-positive patients and 27% of others. VE is underestimated by 10% if non-testing occurs in 32% of unvaccinated influenza-negative patients and 18% of others.

CONCLUSIONS

Although differential clinical testing by vaccine receipt and influenza positivity may produce errors in estimated VE, bias in testing would have to be substantial and overall proportion of patients tested would have to be small to result in a meaningful difference in VE.

The effect of sex on responses to influenza vaccines

The poor uptake and limited effectiveness of seasonal influenza vaccines mean that influenza continues to create a significant burden of disease. It has been hypothesized that sex differences are present in responses to seasonal influenza vaccines, and that these differences may contribute to this poor vaccine success. This has led to the suggestion that vaccines should be tailored to an individual's biological sex. However, studies in this field are often low quality. Comprehensive analysis of the available literature reveals that there is insufficient evidence to support sex differences in vaccine immunogenicity, effectiveness, or efficacy. Nonetheless, differences in vaccine safety are consistently observed, with females reporting adverse events following immunization more frequently than males. Bias introduced by gender differences in passive reporting of adverse effects may underlie this phenomenon. Highly controlled studies are required in future before any conclusions can be made about potential sex differences in response to seasonal influenza vaccines.

Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine

BACKGROUND

Vaccines are needed to prevent coronavirus disease 2019 (Covid-19) and to protect persons who are at high risk for complications. The mRNA-1273 vaccine is a lipid nanoparticle-encapsulated mRNA-based vaccine that encodes the prefusion stabilized full-length spike protein of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes Covid-19.

METHODS

This phase 3 randomized, observer-blinded, placebo-controlled trial was conducted at 99 centers across the United States. Persons at high risk for SARS-CoV-2 infection or its complications were randomly assigned in a 1:1 ratio to receive two intramuscular injections of mRNA-1273 (100 μg) or placebo 28 days apart. The primary end point was prevention of Covid-19 illness with onset at least 14 days after the second injection in participants who had not previously been infected with SARS-CoV-2.

RESULTS

The trial enrolled 30,420 volunteers who were randomly assigned in a 1:1 ratio to receive either vaccine or placebo (15,210 participants in each group). More than 96% of participants received both injections, and 2.2% had evidence (serologic, virologic, or both) of SARS-CoV-2 infection at baseline. Symptomatic Covid-19 illness was confirmed in 185 participants in the placebo group (56.5 per 1000 person-years; 95% confidence interval [CI], 48.7 to 65.3) and in 11 participants in the mRNA-1273 group (3.3 per 1000 person-years; 95% CI, 1.7 to 6.0); vaccine efficacy was 94.1% (95% CI, 89.3 to 96.8%; P<0.001). Efficacy was similar across key secondary analyses, including assessment 14 days after the first dose, analyses that included participants who had evidence of SARS-CoV-2 infection at baseline, and analyses in participants 65 years of age or older. Severe Covid-19 occurred in 30 participants, with one fatality; all 30 were in the placebo group. Moderate, transient reactogenicity after vaccination occurred more frequently in the mRNA-1273 group. Serious adverse events were rare, and the incidence was similar in the two groups.

CONCLUSIONS

The mRNA-1273 vaccine showed 94.1% efficacy at preventing Covid-19 illness, including severe disease. Aside from transient local and systemic reactions, no safety concerns were identified. (Funded by the Biomedical Advanced Research and Development Authority and the National Institute of Allergy and Infectious Diseases; COVE ClinicalTrials.gov number, NCT04470427.).

Effectiveness of trivalent inactivated influenza vaccines in children during 2017-2018 season in Korea: Comparison of test-negative analysis by rapid and RT-PCR influenza tests

Objectives

In Korea, the National Immunization Program provided trivalent inactivated influenza vaccines (IIV3) to all children aged 6–59 months during the 2017–2018 season. In this study, we aimed to evaluate the vaccine effectiveness (VE) of IIV3 in children during the 2017–2018 season.

Methods

Children aged 6–59 months who were tested for influenza for their acute respiratory illness in four hospitals during the 2017–2018 influenza season were included. We estimated the VE of IIV3 by test-negative case-control design based on the rapid influenza diagnostic test (RIDT) or reverse transcription polymerase chain reaction (RT-PCR) test results.

Results

A total of 4738 children were included in this study. The number of laboratory-confirmed influenza cases was 845 (17.8%), and there were 478 cases of influenza A and 362 cases of influenza B. The adjusted VE based on RT-PCR was 53.4% (95% CI, 25.3–70.5) against any influenza, 68.8% (95% CI, 38.7–84.1) against influenza A, and 29.7% (95% CI, −35.1 to 61.8) for influenza B. The adjusted VE based on RIDT was 14.8% (95% CI, −4.4 to 30.0) against any influenza, 24.2% (95% CI, 3.1–40.2) against influenza A, and −5.1% (95% CI, −42.6 to 21.4) against influenza B. Age-specific VE based on RT-PCR against any influenza was 44.1% (95% CI, −0.2 to 67.8) in children aged 6 months to 2 years and 59.3% (95% CI, 8.8–81.9) in children aged 3–<5 years.

Conclusion

Our results suggest moderate protection (53.4%) of IIV3 against RT-PCR laboratory-confirmed influenza in children in the 2017–2018 influenza season. However, the RIDT hampered the validity to assess VE during influenza season. Caution is needed when interpreting an RIDT-based test negative design influenza VE study.

Effectiveness of the quadrivalent inactivated influenza vaccine in Japan during the 2015-2016 season: A test-negative case-control study comparing the results by real time PCR, virus isolation

BACKGROUND

We estimated influenza vaccine effectiveness (VE) in 2015-2016 season against medically attended, laboratory-confirmed influenza, when quadrivalent inactivated vaccine (IIV4) was first introduced in Japan, using test-negative case-control design. Influenza A(H1N1)pdm09 cocirculated with B/Yamagata and B/Victoria during the study period in Japan.

METHOD

We based our case definition on two laboratory tests, real-time reverse transcription polymerase chain reaction (RT PCR), and virus isolation and compared VEs based on these tests. In addition, VE was evaluated by rapid diagnostic test (RDT). Nasopharyngeal swabs were collected from outpatients who visited clinics with influenza-like illness (ILIs) in Hokkaido, Niigata, Gunma and Nagasaki prefectures.

RESULTS

Among 713 children and adults enrolled in this study, 578 were influenza positive by RT PCR including, 392 influenza A and 186 influenza B, while 135 were tested negative controls. The adjusted VE by RT PCR for all ages against any influenza was low protection of 36.0% (95% confidence interval [CI], 3.1% to 58.6%), for influenza A was 30.0% (95% CI: -10.0% to 55.5%), and influenza B was moderate 50.2% (95% CI: 13.3% to 71.4%). Adjusted VE for virus isolation for A(H1N1)pdm09 was 37.1% (95% CI: 1.7% to 59.7%), Yamagata lineage 51.3% (95% CI: 6.4% to 74.7%) and Victoria lineage 21.3% (95% CI: -50.0% to 58.9%). VE was highest and protective in 0-5 years old group against any influenza and influenza A and B/Yamagata, but the protective effect was not observed for other age groups and B/Victoria. RDT demonstrated concordant results with RT PCR and virus isolation. Sequencing of hemagglutinin gene showed that all A(H1N1)pdm09 belong to clade 6B including 31 strains (88.6%), which belong to clade 6B.1 possessing S162N mutations that may alter antigenicity and affect VE for A(H1N1)pdm09.

CONCLUSIONS

IIV4 influenza vaccine during 2015-2016 was effective against A(H1N1)pdm09 and the two lineages of type B. Younger children was more protected than older children and adults by vaccination.

Use of self-reported vaccination status can bias vaccine effectiveness estimates from test-negative studies

Introduction

A number of national/multi-national networks provide annual estimates of influenza vaccine effectiveness (VE) based on the test-negative design. Most of these networks use subject self-reports to define influenza vaccination history. In this study, we used simulations to estimate the degree to which self-reported vaccination status can bias test-negative VE estimates.

Methods

We simulated a population whose members are at risk for acute respiratory illness (ARI) due to influenza and for ARI due to other respiratory pathogens. Vaccination was assumed to reduce the risk of influenza but not of non-influenza ARI. We simulated a range of possible values for VE and for vaccine coverage. Across simulations, we varied the sensitivity and specificity of self-reported vaccination status relative to true vaccination. We estimated bias as the percent difference in VE in the presence of misclassification relative to true simulated VE.

Results

Assuming self-report has sensitivity of 95% and specificity of 90%, estimated VE underestimated true VE by 16% (95% confidence interval, 4–30%). Decreasing specificity of self-reports resulted in greater bias than decreasing sensitivity of self-reports. Bias also increased as vaccine coverage decreased.

Conclusions

The use of self-reported influenza vaccination history can meaningfully bias influenza VE in test-negative studies. Researchers using test-negative designs should attempt to supplement or validate self-reported vaccination history using additional data sources.

Characterising seasonal influenza epidemiology using primary care surveillance data

Understanding the epidemiology of seasonal influenza is critical for healthcare resource allocation and early detection of anomalous seasons. It can be challenging to obtain high-quality data of influenza cases specifically, as clinical presentations with influenza-like symptoms may instead be cases of one of a number of alternate respiratory viruses. We use a new dataset of confirmed influenza virological data from 2011-2016, along with high-quality denominators informing a hierarchical observation process, to model seasonal influenza dynamics in New South Wales, Australia. We use approximate Bayesian computation to estimate parameters in a climate-driven stochastic epidemic model, including the basic reproduction number R₀, the proportion of the population susceptible to the circulating strain at the beginning of the season, and the probability an infected individual seeks treatment. We conclude that R₀ and initial population susceptibility were strongly related, emphasising the challenges of identifying these parameters. Relatively high R₀ values alongside low initial population susceptibility were among the results most consistent with these data. Our results reinforce the importance of distinguishing between R₀ and the effective reproduction number (R_e) in modelling studies.

When patients present to their doctor with influenza-like symptoms, they may have influenza, or some other respiratory virus. The only way to discriminate between these viruses is with an expensive test, which is not performed in many cases. Additionally, results other than influenza may not be reported. This means that it can be difficult to determine how much influenza is circulating in the population each season. We used a unique dataset of confirmed influenza with denominators to fit models for seasonal influenza in New South Wales, Australia. Knowing the denominators allowed us to estimate population level trends. We found that the relationship between influenza transmission rates and immunity due to previous infections was critical, with relatively high transmission corresponding to substantial preexisting immunity likely. This existing immunity is critical to understanding and effectively modeling influenza dynamics.

Influenza vaccine failure: failure to protect or failure to understand?

INTRODUCTION

I propose that influenza vaccine failure be defined as receipt of a properly stored and administered vaccine with the subsequent development of documented influenza. Several mechanisms of vaccine failure occur and can - sometimes in combination - lead to what is termed 'vaccine failure.' Influenza vaccine failure occurs for many reasons, many of which are not true failures of the vaccine (e.g. improper vaccine storage/handling).

AREAS COVERED

In this article, I discuss common causes of 'vaccine failure' that are appropriately or inappropriately attributed to vaccines. This includes host, pathogen, vaccine, and study design issues such as genetic restriction of immune response; failure to store, handle, and administer vaccine properly; issues of immunosuppression and immunosenescence; apparent but false vaccine failure; time-mediated failure; etc.

EXPERT COMMENTARY

A proper framework and nosology for vaccine failure informs discussion about influenza vaccine efficacy and prevents misperceptions that in turn affect vaccine uptake. Influenza vaccine can only provide maximum protection to the extent that the circulating and vaccine strains closely match; the vaccine is stored, handled, and administered properly and within a time frame to result in development of protective levels of immunity; and it is administered to a host capable of immunologically responding with protective immune responses.

Influenza vaccine effectiveness in older adults compared with younger adults over five seasons

BACKGROUND

There have been inconsistent reports of decreased vaccine effectiveness (VE) against influenza viruses among older adults (aged ≥ 65 years) compared with younger adults in the United States. A direct comparison of VE over multiple seasons is needed to assess the consistency of these observations.

METHODS

We performed a pooled analysis of VE over 5 seasons among adults aged ≥ 18 years who were systematically enrolled in the U.S. Flu VE Network. Outpatients with medically-attended acute respiratory illness (cough with illness onset ≤ 7 days prior to enrollment) were tested for influenza by reverse transcription polymerase chain reaction. We compared differences in VE and vaccine failures among older adult age group (65-74, ≥75, and ≥ 65 years) to adults aged 18-49 years by influenza type and subtype using interaction terms to test for statistical significance and stratified by prior season vaccination status.

RESULTS

Analysis included 20,022 adults aged ≥ 18 years enrolled during the 2011-12 through 2015-16 influenza seasons; 4,785 (24%) tested positive for influenza. VE among patients aged ≥ 65 years was not significantly lower than VE among patients aged 18-49 years against any subtype with no significant interaction of age and vaccination. VE against A(H3N2) viruses was 14% (95% confidence interval [CI] -14% to 36%) for adults ≥ 65 years and 21% (CI 9-32%) for adults 18-49 years. VE against A(H1N1)pdm09 was 49% (95% CI 22-66%) for adults ≥ 65 years and 48% (95% CI 41-54%) for adults 18-49 years and against B viruses was 62% (95% CI 44-74%) for adults ≥ 65 years and 55% (95% CI 45-63%) for adults 18-49 years. There was no significant interaction of age and vaccination for separate strata of prior vaccination status.

CONCLUSIONS

Over 5 seasons, influenza vaccination provided similar levels of protection among older and younger adults, with lower levels of protection against influenza A(H3N2) in all ages.

A comparison of the test-negative and the traditional case-control study designs for estimation of influenza vaccine effectiveness under nonrandom vaccination

Background

As annual influenza vaccination is recommended for all U.S. persons aged 6 months or older, it is unethical to conduct randomized clinical trials to estimate influenza vaccine effectiveness (VE). Observational studies are being increasingly used to estimate VE. We developed a probability model for comparing the bias and the precision of VE estimates from two case-control designs: the traditional case-control (TCC) design and the test-negative (TN) design. In both study designs, acute respiratory illness (ARI) patients seeking medical care testing positive for influenza infection are considered cases. In the TN design, ARI patients seeking medical care who test negative serve as controls, while in the TCC design, controls are randomly selected individuals from the community who did not contract an ARI.

Methods

Our model assigns each study participant a covariate corresponding to the person’s health status. The probabilities of vaccination and of contracting influenza and non-influenza ARI depend on health status. Hence, our model allows non-random vaccination and confounding. In addition, the probability of seeking care for ARI may depend on vaccination and health status. We consider two outcomes of interest: symptomatic influenza (SI) and medically-attended influenza (MAI).

Results

If vaccination does not affect the probability of non-influenza ARI, then VE estimates from TN studies usually have smaller bias than estimates from TCC studies. We also found that if vaccinated influenza ARI patients are less likely to seek medical care than unvaccinated patients because the vaccine reduces symptoms’ severity, then estimates of VE from both types of studies may be severely biased when the outcome of interest is SI. The bias is not present when the outcome of interest is MAI.

Conclusions

The TN design produces valid estimates of VE if (a) vaccination does not affect the probabilities of non-influenza ARI and of seeking care against influenza ARI, and (b) the confounding effects resulting from non-random vaccination are similar for influenza and non-influenza ARI. Since the bias of VE estimates depends on the outcome against which the vaccine is supposed to protect, it is important to specify the outcome of interest when evaluating the bias.

Influenza Vaccine Effectiveness in the United States during the 2015-2016 Season

BACKGROUND

The A(H1N1)pdm09 virus strain used in the live attenuated influenza vaccine was changed for the 2015-2016 influenza season because of its lack of effectiveness in young children in 2013-2014. The Influenza Vaccine Effectiveness Network evaluated the effect of this change as part of its estimates of influenza vaccine effectiveness in 2015-2016.

METHODS

We enrolled patients 6 months of age or older who presented with acute respiratory illness at ambulatory care clinics in geographically diverse U.S. sites. Using a test-negative design, we estimated vaccine effectiveness as (1-OR)×100, in which OR is the odds ratio for testing positive for influenza virus among vaccinated versus unvaccinated participants. Separate estimates were calculated for the inactivated vaccines and the live attenuated vaccine.

RESULTS

Among 6879 eligible participants, 1309 (19%) tested positive for influenza virus, predominantly for A(H1N1)pdm09 (11%) and influenza B (7%). The effectiveness of the influenza vaccine against any influenza illness was 48% (95% confidence interval [CI], 41 to 55; P<0.001). Among children 2 to 17 years of age, the inactivated influenza vaccine was 60% effective (95% CI, 47 to 70; P<0.001), and the live attenuated vaccine was not observed to be effective (vaccine effectiveness, 5%; 95% CI, -47 to 39; P=0.80). Vaccine effectiveness against A(H1N1)pdm09 among children was 63% (95% CI, 45 to 75; P<0.001) for the inactivated vaccine, as compared with -19% (95% CI, -113 to 33; P=0.55) for the live attenuated vaccine.

CONCLUSIONS

Influenza vaccines reduced the risk of influenza illness in 2015-2016. However, the live attenuated vaccine was found to be ineffective among children in a year with substantial inactivated vaccine effectiveness. Because the 2016-2017 A(H1N1)pdm09 strain used in the live attenuated vaccine was unchanged from 2015-2016, the Advisory Committee on Immunization Practices made an interim recommendation not to use the live attenuated influenza vaccine for the 2016-2017 influenza season. (Funded by the Centers for Disease Control and Prevention and the National Institutes of Health.).

Case-control vaccine effectiveness studies: Data collection, analysis and reporting results

The case-control methodology is frequently used to evaluate vaccine effectiveness post-licensure. The results of such studies provide important insight into the level of protection afforded by vaccines in a 'real world' context, and are commonly used to guide vaccine policy decisions. However, the potential for bias and confounding are important limitations to this method, and the results of a poorly conducted or incorrectly interpreted case-control study can mislead policies. In 2012, a group of experts met to review recent experience with case-control studies evaluating vaccine effectiveness; we summarize the recommendations of that group regarding best practices for data collection, analysis, and presentation of the results of case-control vaccine effectiveness studies. Vaccination status is the primary exposure of interest, but can be challenging to assess accurately and with minimal bias. Investigators should understand factors associated with vaccination as well as the availability of documented vaccination status in the study context; case-control studies may not be a valid method for evaluating vaccine effectiveness in settings where many children lack a documented immunization history. To avoid bias, it is essential to use the same methods and effort gathering vaccination data from cases and controls. Variables that may confound the association between illness and vaccination are also important to capture as completely as possible, and where relevant, adjust for in the analysis according to the analytic plan. In presenting results from case-control vaccine effectiveness studies, investigators should describe enrollment among eligible cases and controls as well as the proportion with no documented vaccine history. Emphasis should be placed on confidence intervals, rather than point estimates, of vaccine effectiveness. Case-control studies are a useful approach for evaluating vaccine effectiveness; however careful attention must be paid to the collection, analysis and presentation of the data in order to best inform evidence-based vaccine policies.

Prevention and Control of Seasonal Influenza with Vaccines

This report updates the 2015-16 recommendations of the Advisory Committee on Immunization Practices (ACIP) regarding the use of seasonal influenza vaccines (Grohskopf LA, Sokolow LZ, Olsen SJ, Bresee JS, Broder KR, Karron RA. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices, United States, 2015-16 influenza season. MMWR Morb Mortal Wkly Rep 2015;64:818-25). Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. For the 2016-17 influenza season, inactivated influenza vaccines (IIVs) will be available in both trivalent (IIV3) and quadrivalent (IIV4) formulations. Recombinant influenza vaccine (RIV) will be available in a trivalent formulation (RIV3). In light of concerns regarding low effectiveness against influenza A(H1N1)pdm09 in the United States during the 2013-14 and 2015-16 seasons, for the 2016-17 season, ACIP makes the interim recommendation that live attenuated influenza vaccine (LAIV4) should not be used. Vaccine virus strains included in the 2016-17 U.S. trivalent influenza vaccines will be an A/California/7/2009 (H1N1)-like virus, an A/Hong Kong/4801/2014 (H3N2)-like virus, and a B/Brisbane/60/2008-like virus (Victoria lineage). Quadrivalent vaccines will include an additional influenza B virus strain, a B/Phuket/3073/2013-like virus (Yamagata lineage).Recommendations for use of different vaccine types and specific populations are discussed. A licensed, age-appropriate vaccine should be used. No preferential recommendation is made for one influenza vaccine product over another for persons for whom more than one licensed, recommended product is otherwise appropriate. This information is intended for vaccination providers, immunization program personnel, and public health personnel. Information in this report reflects discussions during public meetings of ACIP held on October 21, 2015; February 24, 2016; and June 22, 2016. These recommendations apply to all licensed influenza vaccines used within Food and Drug Administration-licensed indications, including those licensed after the publication date of this report. Updates and other information are available at CDC's influenza website (http://www.cdc.gov/flu). Vaccination and health care providers should check CDC's influenza website periodically for additional information.

Validity of the estimates of oral cholera vaccine effectiveness derived from the test-negative design

BACKGROUND

The test-negative design (TND) has emerged as a simple method for evaluating vaccine effectiveness (VE). Its utility for evaluating oral cholera vaccine (OCV) effectiveness is unknown. We examined this method's validity in assessing OCV effectiveness by comparing the results of TND analyses with those of conventional cohort analyses.

METHODS

Randomized controlled trials of OCV were conducted in Matlab (Bangladesh) and Kolkata (India), and an observational cohort design was used in Zanzibar (Tanzania). For all three studies, VE using the TND was estimated from the odds ratio (OR) relating vaccination status to fecal test status (Vibrio cholerae O1 positive or negative) among diarrheal patients enrolled during surveillance (VE= (1-OR)×100%). In cohort analyses of these studies, we employed the Cox proportional hazard model for estimating VE (=1-hazard ratio)×100%).

RESULTS

OCV effectiveness estimates obtained using the TND (Matlab: 51%, 95% CI:37-62%; Kolkata: 67%, 95% CI:57-75%) were similar to the cohort analyses of these RCTs (Matlab: 52%, 95% CI:43-60% and Kolkata: 66%, 95% CI:55-74%). The TND VE estimate for the Zanzibar data was 94% (95% CI:84-98%) compared with 82% (95% CI:58-93%) in the cohort analysis. After adjusting for residual confounding in the cohort analysis of the Zanzibar study, using a bias indicator condition, we observed almost no difference in the two estimates.

CONCLUSION

Our findings suggest that the TND is a valid approach for evaluating OCV effectiveness in routine vaccination programs.

Technical guidelines for the application of seasonal influenza vaccine in China (2014-2015)

Influenza, caused by the influenza virus, is a respiratory infectious disease that can severely affect human health. Influenza viruses undergo frequent antigenic changes, thus could spread quickly. Influenza causes seasonal epidemics and outbreaks in public gatherings such as schools, kindergartens, and nursing homes. Certain populations are at risk for severe illness from influenza, including pregnant women, young children, the elderly, and people in any ages with certain chronic diseases.

Can the rolling cross-sectional survey design be used to estimate the effectiveness of influenza vaccines?

INTRODUCTION

Observational studies of influenza vaccine effectiveness often study persons seeking medical care for acute respiratory infection (ARI). We conducted a pilot study to determine if vaccine effectiveness could be estimated in the general population with a novel rolling cross-sectional survey sampling design and laboratory confirmation of influenza.

METHODS

Cross-sectional samples were selected weekly from defined populations in Marshfield, Wisconsin and Monroe County, New York from January through April, 2011 (12 weeks). Persons were telephoned and asked about the occurrence of ARI in the past week. Nasal and throat swabs were obtained from consenting individuals with ARI and tested by real-time reverse transcription polymerase chain reaction (RT-PCR). Vaccine effectiveness (VE) was defined as (100×[1-OR]) for vaccination in a logistic regression model that adjusted for age, calendar week, and site. The comparison group included all study participants without RT-PCR confirmed influenza, including those who were not ill.

RESULTS

Study personnel contacted 9537 (62%) of 15,303 persons sampled; the primary analysis included 5678 subjects. Of these, 193 (3%) reported an ARI and agreed to be tested for influenza; 13 (7%) were influenza positive. The adjusted effectiveness of the influenza vaccine was 1% (95% confidence limits -239-70%).

CONCLUSIONS

The rolling cross-sectional design is methodologically feasible and may be useful as a complement to clinic-based VE studies. This pilot study did not have sufficient power to detect significant vaccine effectiveness during a mild influenza season, but this approach may facilitate rapid estimation of VE in a pandemic setting when normal patterns of health care utilization are disrupted.

A probability model for evaluating the bias and precision of influenza vaccine effectiveness estimates from case-control studies

As influenza vaccination is now widely recommended, randomized clinical trials are no longer ethical in many populations. Therefore, observational studies on patients seeking medical care for acute respiratory illnesses (ARIs) are a popular option for estimating influenza vaccine effectiveness (VE). We developed a probability model for evaluating and comparing bias and precision of estimates of VE against symptomatic influenza from two commonly used case-control study designs: the test-negative design and the traditional case-control design. We show that when vaccination does not affect the probability of developing non-influenza ARI then VE estimates from test-negative design studies are unbiased even if vaccinees and non-vaccinees have different probabilities of seeking medical care against ARI, as long as the ratio of these probabilities is the same for illnesses resulting from influenza and non-influenza infections. Our numerical results suggest that in general, estimates from the test-negative design have smaller bias compared to estimates from the traditional case-control design as long as the probability of non-influenza ARI is similar among vaccinated and unvaccinated individuals. We did not find consistent differences between the standard errors of the estimates from the two study designs.

Trends in racial/ethnic disparities in influenza vaccination coverage among adults during the 2007-08 through 2011-12 seasons

BACKGROUND

Annual influenza vaccination is recommended for all persons aged ≥6 months. The objective of this study was to assess trends in racial/ethnic disparities in influenza vaccination coverage among adults in the United States.

METHODS

We analyzed data from the 2007-2012 National Health Interview Survey (NHIS) and Behavioral Risk Factor Surveillance System (BRFSS) using Kaplan-Meier survival analysis to assess influenza vaccination coverage by age, presence of medical conditions, and racial/ethnic groups during the 2007-08 through 2011-12 seasons.

RESULTS

During the 2011-12 season, influenza vaccination coverage was significantly lower among non-Hispanic blacks and Hispanics compared with non-Hispanic whites among most of the adult subgroups, with smaller disparities observed for adults age 18-49 years compared with other age groups. Vaccination coverage for non-Hispanic white, non-Hispanic black, and Hispanic adults increased significantly from the 2007-08 through the 2011-12 season for most of the adult subgroups based on the NHIS (test for trend, P < .05). Coverage gaps between racial/ethnic minorities and non-Hispanic whites persisted at similar levels from the 2007-08 through the 2011-12 seasons, with similar results from the NHIS and BRFSS.

CONCLUSIONS

Influenza vaccination coverage among most racial/ethnic groups increased from the 2007-08 through the 2011-12 seasons, but substantial racial and ethnic disparities remained in most age groups. Targeted efforts are needed to improve coverage and reduce these disparities.

Child, household, and caregiver characteristics associated with hospitalization for influenza among children 6-59 months of age: an emerging infections program study

BACKGROUND

Young children are at increased risk of severe outcomes from influenza illness, including hospitalization. We conducted a case-control study to identify risk factors for influenza-associated hospitalizations among children in US Emerging Infections Program sites.

METHODS

Cases were children 6-59 months of age hospitalized for laboratory-confirmed influenza infections during 2005-2008. Age- and zip-code-matched controls were enrolled. Data on child, caregiver and household characteristics were collected from parents and medical records. Conditional logistic regression was used to identify independent risk factors for hospitalization.

RESULTS

We enrolled 290 (64%) of 454 eligible cases and 1089 (49%) of 2204 eligible controls. Risk for influenza hospitalization increased with maternal age <26 years [odds ratio (OR): 1.8, 95% confidence interval (CI): 1.1-2.9]; household income below the poverty threshold (OR: 2.2, 95% CI: 1.4-3.6); smoking by >50% of household members (OR: 2.9, 95% CI: 1.4-6.6); lack of household influenza vaccination (OR: 1.8, 95% CI: 1.2-2.5) and presence of chronic illnesses, including hematologic/oncologic (OR: 11.8, 95% CI: 4.5-31.0), pulmonary (OR: 2.9, 95% CI: 1.9-4.4) and neurologic (OR: 3.8, 95% CI: 1.6-9.2) conditions. Full influenza immunization decreased the risk among children 6-23 months of age (OR: 0.5, 95% CI: 0.3-0.9) but not among those 24-59 months of age (OR: 1.5, 95% CI: 0.8-3.0; P value for difference = 0.01).

CONCLUSIONS

Chronic illnesses, young maternal age, poverty, household smoking and lack of household influenza vaccination increased the risk of influenza hospitalization. These characteristics may help providers to identify young children who are at greatest risk for severe outcomes from influenza illness.

Influenza vaccine effectiveness in the 2011-2012 season: protection against each circulating virus and the effect of prior vaccination on estimates

BACKGROUND

Each year, the US Influenza Vaccine Effectiveness Network examines the effectiveness of influenza vaccines in preventing medically attended acute respiratory illnesses caused by influenza.

METHODS

Patients with acute respiratory illnesses of ≤ 7 days' duration were enrolled at ambulatory care facilities in 5 communities. Specimens were collected and tested for influenza by real-time reverse-transcriptase polymerase chain reaction. Receipt of influenza vaccine was defined based on documented evidence of vaccination in medical records or immunization registries. Vaccine effectiveness was estimated in adjusted logistic regression models by comparing the vaccination coverage in those who tested positive for influenza with those who tested negative.

RESULTS

The 2011-2012 season was mild and peaked late, with circulation of both type A viruses and both lineages of type B. Overall adjusted vaccine effectiveness was 47% (95% confidence interval [CI], 36-56) in preventing medically attended influenza; vaccine effectiveness was 65% (95% CI, 44-79) against type A (H1N1) pdm09 but only 39% (95% CI, 23-52) against type A (H3N2). Estimates of vaccine effectiveness against both type B lineages were similar (overall, 58%; 95% CI, 35-73). An apparent negative effect of prior year vaccination on current year effectiveness estimates was noted, particularly for A (H3N2) outcomes.

CONCLUSIONS

Vaccine effectiveness in the 2011-2012 season was modest overall, with lower effectiveness against the predominant A (H3N2) virus. This may be related to antigenic drift, but past history of vaccination might also play a role.

Does outpatient laboratory testing represent influenza burden and distribution in a rural state?

BACKGROUND

Laboratory testing results are often used to monitor influenza illness in populations, but results may not be representative of illness burden and distribution, especially in populations that are geographically, socioeconomically, and racially/ethnically diverse.

OBJECTIVES

Descriptive epidemiology and chi-square analyses using demographic, geographic, and medical condition prevalence comparisons were employed to assess whether a group of individuals with outpatient laboratory-confirmed influenza illness during September-November 2009 represented the burden and distribution of influenza illness in New Mexico (NM).

PATIENTS/METHODS

The outpatient group was identified via random selection from those with positive influenza tests at NM laboratories. Comparison groups included those with laboratory-confirmed H1N1-related influenza hospitalization and death identified via prospective active statewide surveillance, those with self-reported influenza-like illness (ILI) identified through random digit dialing, and the NM population.

RESULTS

This analysis included 334 individuals with outpatient laboratory-confirmed influenza, 888 individuals with laboratory-confirmed H1N1-related hospitalization, 39 individuals with laboratory-confirmed H1N1-related death, 334 individuals with ILI, and NM population data (N = 2,036,112). The outpatient laboratory-confirmed group had a different distribution of demographic and geographic factors, as well as prevalence of certain medical conditions as compared to the groups of laboratory-confirmed H1N1-related hospitalization and death, the ILI group, and the NM population.

CONCLUSIONS

The outpatient laboratory-confirmed group may reflect provider testing practices and potentially healthcare-seeking behavior and access to care, rather than influenza burden and distribution in NM during the H1N1 pandemic.

Influenza vaccine effectiveness in the community and the household

BACKGROUND

There is a recognized need to determine influenza vaccine effectiveness on an annual basis and a long history of studying respiratory illnesses in households.

METHODS

We recruited 328 households with 1441 members, including 839 children, and followed them during the 2010-2011 influenza season. Specimens were collected from subjects with reported acute respiratory illnesses and tested by real-time reverse transcriptase polymerase chain reaction. Receipt of influenza vaccine was defined based on documented evidence of vaccination in medical records or an immunization registry. The effectiveness of 2010-2011 influenza vaccination in preventing laboratory-confirmed influenza was estimated using Cox proportional hazards models adjusted for age and presence of high-risk condition, and stratified by prior season (2009-2010) vaccination status.

RESULTS

Influenza was identified in 78 (24%) households and 125 (9%) individuals; the infection risk was 8.5% in the vaccinated and 8.9% in the unvaccinated (P = .83). Adjusted vaccine effectiveness in preventing community-acquired influenza was 31% (95% confidence interval [CI], -7% to 55%). In vaccinated subjects with no evidence of prior season vaccination, significant protection (62% [95% CI, 17%-82%]) against community-acquired influenza was demonstrated. Substantially lower effectiveness was noted among subjects who were vaccinated in both the current and prior season. There was no evidence that vaccination prevented household transmission once influenza was introduced; adults were at particular risk despite vaccination.

CONCLUSIONS

Vaccine effectiveness estimates were lower than those demonstrated in other observational studies carried out during the same season. The unexpected findings of lower effectiveness with repeated vaccination and no protection given household exposure require further study.

Estimating the influenza vaccine effectiveness against medically attended influenza in clinical settings: a hospital-based case-control study with a rapid diagnostic test in Japan

Background

Influenza vaccine effectiveness (VE) studies are usually conducted by specialized agencies and require time and resources. The objective of this study was to estimate the influenza VE against medically attended influenza using a test-negative case-control design with rapid influenza diagnostic tests (RIDT) in a clinical setting.

Methods

A prospective study was conducted at a community hospital in Nagasaki, western Japan during the 2010/11 influenza season. All outpatients aged 15 years and older with influenza-like illnesses (ILI) who had undergone RIDT were enrolled. A test-negative case-control design was applied to estimate the VEs: the cases were ILI patients with positive RIDT results and the controls were ILI patients with negative RIDT results. Information on patient characteristics, including vaccination histories, was collected using questionnaires and medical records.

Results

Between December 2010 and April 2011, 526 ILI patients were tested with RIDT, and 476 were eligible for the analysis. The overall VE estimate against medically attended influenza was 47.6%, after adjusting for the patients' age groups, presence of chronic conditions, month of visit, and smoking and alcohol use. The seasonal influenza vaccine reduced the risk of medically attended influenza by 60.9% for patients less than 50 years of age, but a significant reduction was not observed for patients 50 years of age and older. A sensitivity analysis provided similar figures.

Conclusion

The test-negative case-control study using RIDT provided moderate influenza VE consistent with other reports. Utilizing the commonly used RIDT to estimate VE provides rapid assessment of VE; however, it may require validation with more specific endpoint.

Effectiveness of seasonal influenza vaccines against influenza-associated illnesses among US military personnel in 2010-11: a case-control approach

INTRODUCTION

Following the 2009 influenza A/H1N1 (pH1N1) pandemic, both seasonal and pH1N1 viruses circulated in the US during the 2010-2011 influenza season; influenza vaccine effectiveness (VE) may vary between live attenuated (LAIV) and trivalent inactivated (TIV) vaccines as well as by virus subtype.

MATERIALS AND METHODS

Vaccine type and virus subtype-specific VE were determined for US military active component personnel for the period of September 1, 2010 through April 30, 2011. Laboratory-confirmed influenza-related medical encounters were compared to matched individuals with a non-respiratory illness (healthy controls), and unmatched individuals who experienced a non-influenza respiratory illness (test-negative controls). Odds ratios (OR) and VE estimates were calculated overall, by vaccine type and influenza subtype.

RESULTS

A total of 603 influenza cases were identified. Overall VE was relatively low and similar regardless of whether healthy controls (VE = 26%, 95% CI: -1 to 45) or test-negative controls (VE = 29%, 95% CI: -6 to 53) were used as comparison groups. Using test-negative controls, vaccine type-specific VE was found to be higher for TIV (53%, 95% CI: 25 to 71) than for LAIV (VE = -13%, 95% CI: -77 to 27). Influenza subtype-specific analyses revealed moderate protection against A/H3 (VE = 58%, 95% CI: 21 to 78), but not against A/H1 (VE = -38%, 95% CI: -211 to 39) or B (VE = 34%, 95% CI: -122 to 80).

CONCLUSION

Overall, a low level of protection against clinically-apparent, laboratory-confirmed, influenza was found for the 2010-11 seasonal influenza vaccines. TIV immunization was associated with higher protection than LAIV, however, no protection against A/H1 was noted, despite inclusion of a pandemic influenza strain as a vaccine component for two consecutive years. Vaccine virus mismatch or lower immunogenicity may have contributed to these findings and deserve further examination in controlled studies. Continued assessment of VE in military personnel is essential in order to better inform vaccination policy decisions.