The impact of repeated vaccination on influenza vaccine effectiveness: a systematic review and meta-analysis

Effectiveness of influenza vaccination in preventing confirmed influenza cases and hospitalizations in Northern Spain, 2023/24 season: A population-based test-negative case-control study

OBJECTIVE

We estimated the influenza vaccination effectiveness (IVE) in preventing medical consultations and hospitalizations due to influenza during the 2023/24 season.

METHODS

Two test-negative case-control studies analyzed patients who consulted primary healthcare or were hospitalized for respiratory symptoms and were tested for influenza by PCR in the 2023/24 season in Navarre, Spain. Influenza vaccination status in the current and previous seasons was compared between confirmed influenza cases and test-negative controls. IVE was calculated as (1-adjusted odds ratio)×100.

RESULTS

Of 3133 hospitalized patients, 529 (17%) were positive for influenza: 71% A/H1N1, 23% A/H3N2, and 7% A nonsubtyped. IVE to prevent hospitalizations was 43% (95%CI: 26%, 56%) overall, 61% (95%CI: 32%, 77%) in people younger than 65 years old and 35% (95%CI: 10%, 53%) in older people. IVE was 48% (95%CI: 30%, 61%) against influenza A/H1N1, and 15% (95%CI: -42%, 49%) against influenza A/H3N2. IVE in people vaccinated only in the previous season was 28% (95%CI: -5%, 51%). Among 417 outpatients, 146 (35%) were confirmed for influenza. IVE to prevent outpatient cases was 49% (95%CI: -9%, 76%) overall and 42% (95%CI: -33%, 74%) against influenza A/H1N1.

CONCLUSIONS

IVE was moderate against influenza A/H1N1 and low against influenza A/H3N2 in the 2023/24 season.

Antibody responses against influenza A decline with successive years of annual influenza vaccination

Influenza vaccine effectiveness and immunogenicity can be compromised with repeated vaccination. We assessed immunological markers in a cohort of healthcare workers (HCW) from six public hospitals around Australia during 2020-2021. Sera were collected pre-vaccination and ~14 and ~180 days post-vaccination and assessed in haemagglutination inhibition assay against egg-grown vaccine and equivalent cell-grown viruses. Responses to vaccination were compared by the number of prior vaccinations. Baseline sera were available for 595 HCW in 2020 and 1031 in 2021. 5% had not been vaccinated during five years prior to enrolment and 55% had been vaccinated every year. Post-vaccination titres for all vaccine antigens were lowest among HCW vaccinated in all 5-prior years and highest among HCW with 0 or 1 prior vaccinations, even after adjustment. This was observed for both influenza A subtypes and was dependent on pre-vaccination titre. Expanded cohorts are needed to better understand how this translates to vaccine effectiveness.

Immune imprinting: The persisting influence of the first antigenic encounter with rapidly evolving viruses

Immune imprinting is a phenomenon that stems from the fundamentals of immunological memory. Upon recurrent exposures to an evolving pathogen, the immune system must weigh the benefits of rapidly recalling established antibody repertoires with greater affinity to the initial variant or invest additional time and energy in producing de novo responses specific to the emerging variant. In this review, we delve into the mechanistic complexities of immune imprinting and its role in shaping subsequent immune responses, both de novo and recall, against rapidly evolving respiratory viruses such as influenza and coronaviruses. By exploring the duality of immune imprinting, we examine its potential to both enhance or hinder immune protection against disease, while emphasizing the role of host and viral factors. Finally, we explore how different vaccine platforms may affect immune imprinting and comment on vaccine strategies that can favor de novo variant-specific antibody responses.

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 Network (2011-2012 to 2018-2019 seasons), we found that repeat vaccinees were vaccinated earlier in a season by 1 week. After accounting for waning VE, we determined that repeat vaccinees were still more likely to test positive for A(H3N2) (odds ratio, 1.11; 95% CI, 1.02-1.21) but not influenza B or A(H1N1). We documented clinical infection influenced individuals' decision to vaccinate in the following season while protecting against clinical infection of the same type/subtype. However, adjusting for recent documented clinical infections did not strongly influence the estimated effect of prior-season vaccination. In contrast, we found that adjusting for subclinical or undocumented infection could theoretically attenuate this effect. Additional investigation is needed to determine the impact of subclinical infections on vaccine effectiveness.

[Determinants of influenza vaccination in people over 65 years of age: A systematic review]

Low influenza vaccination rates among specific risk groups contribute to the burden of disease and remain a major public health challenge. Therefore, the purpose of this review was to provide the most recent evidence on the determinants of influenza vaccination in people over 65years of age. During the review process, we followed the recommendations for PRISMA systematic reviews. The bibliographic search was carried out in databases Cumulative Index to Nursing and Allied Health Literature (CINAHL), SCOPUS, MedLine / PubMed, Cochrane, Google Scholar, with free and controlled language and the search terms MeSh: Vaccination Coverage, Health Communication, Aged, Influenza Vaccines by combining them with the Boolean operators AND and OR. The search was limited to articles published between 2019-2024. Seven original research articles were included, of which observational and cross-sectional studies were identified as the main type of study. The topic of study represents education, the recruitment of the vulnerable population and the detection of sociodemographic factors that affect vaccination coverage. The findings of this review suggest that it would be beneficial to take into account communication, education and recruitment as determining factors of influenza vaccination in those over 65years of age. In this sense, effective strategies could be implemented to increase vaccination coverage in the most vulnerable population and improve future vaccination campaigns.

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 influenza vaccination effectiveness in the current season.

METHODS

We established a 5-year randomized placebo-controlled trial of repeated influenza vaccination (Flublok; Sanofi Pasteur) in adults 18-45 years of age. In the first 2 years, participants were randomized to receive vaccine or saline placebo as follows: placebo-placebo (P-P), placebo-vaccine (P-V), or vaccine-vaccine (V-V). Serum samples were collected each year just before vaccination and after 30 and 182 days. A subset of serum samples collected at 5 time points from 95 participants were tested for antibodies against vaccine strains.

RESULTS

From 23 October 2020 through 11 March 2021 we enrolled and randomized 447 adults. Among vaccinated individuals, antibody titers increased between days 0 and 30 against each of the vaccine strains, with smaller increases for repeat vaccinees who on average had higher prevaccination titers in year 2. There were statistically significant differences in the proportions of participants achieving ≥4-fold rises in antibody titer for the repeat vaccinees for influenza A(H1N1), B/Victoria, and B/Yamagata, but not for A(H3N2). Among participants who received vaccination in year 2, there were no significant differences between the P-V and V-V 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 2 years, during which influenza did not circulate, repeat and first-time vaccinees had similar postvaccination geometric mean titers to all 4 vaccine strains, indicative of similar levels of clinical protection. Clinical Trials Registration. NCT04576377.

Antibody responses against influenza A decline with successive years of annual influenza vaccination: results from an Australian Healthcare Worker cohort

Influenza vaccine effectiveness and immunogenicity can be compromised with repeated vaccination. We assessed immunological markers in a cohort of healthcare workers (HCW) from six public hospitals around Australia during 2020-2021. Sera were collected pre-vaccination and ~14 and ~ 180 days post-vaccination and assessed in haemagglutination inhibition assay against egg-grown vaccine and equivalent cell-grown viruses. Responses to vaccination were compared by the number of prior vaccinations. Baseline sera were available for 595 HCW in 2020 and 1031 in 2021. 5% had not been vaccinated during five years prior to enrolment and 55% had been vaccinated every year. Post-vaccination titres for all vaccine antigens were lowest among HCW vaccinated in all 5-prior years and highest among HCW with 0 or 1 prior vaccinations, even after adjustment. This was observed for both influenza A subtypes and was dependent on pre-vaccination titre. Expanded cohorts are needed to better understand how this translates to vaccine effectiveness.

Influenza Epidemiology and Vaccine Effectiveness Following Funded Influenza Vaccine in Queensland, Australia, 2022

BACKGROUND

In 2022, publicly funded influenza vaccine was made available to all residents of Queensland, Australia. This study compared influenza epidemiology in 2022 with previous years (2017-2021) and estimated influenza vaccine effectiveness (VE) during 2022.

METHODS

The study involved a descriptive analysis of influenza notifications and a case-control study to estimate VE. Cases were notifications of laboratory-confirmed influenza, and controls were individuals who were test negative for COVID-19. Cases and controls were matched on age, postcode and specimen collection date. VE against hospitalisation was investigated by matching hospitalised cases to controls. Conditional logistic regression models were adjusted for sex.

RESULTS

In 2022, Queensland experienced an early influenza season onset (April-May) and high case numbers (n = 45,311), compared to the previous 5 years (annual average: 29,364) and 2020-2021 (2020:6047; 2021:301) during the COVID-19 pandemic. Adjusted VE (VEadj) against laboratory-confirmed influenza was 39% (95% confidence interval [CI]: 37-41), highest for children aged 30 months to < 5 years (61%, 95% CI: 49-70) and lowest for adults aged ≥ 65 years (24%, 95% CI: 17-30). VEadj against influenza-associated hospitalisation was 54% (95% CI: 48-59). Among children < 9 years of age, VEadj against laboratory-confirmed influenza (55%, 95% CI: 49-61) and hospitalisation (67%, 95% CI: 39-82) was higher in those who received a complete dose schedule.

CONCLUSION

In Queensland, the 2022 influenza season started earlier than the previous 5 years. VE against influenza notifications varied across age groups. VE estimates against influenza-associated hospitalisation were higher than those against laboratory-confirmed influenza.

Socioeconomic, health-related, and primary care physician characteristics associated with adherence to seasonal influenza vaccination in Manitoba, Canada: A population-wide record-linkage cohort study

OBJECTIVE

There is a lack of published evidence on factors associated with adherence (maintenance of cumulative vaccination) to seasonal influenza vaccination (SIV) in Manitoba, Canada. We sought to assess the associations.

METHODS

A cohort study utilizing Manitoba administrative health databases. Participants received SIV in 2010/11 influenza season, remained registered Manitoba residents and received at least one SIV during the 2011/12‒2019/20 seasons. We dichotomized adherence into "more adherent" (6‒9 SIVs) and "less adherent" (1‒5 SIVs) and used multivariable adjusted generalized estimating equation logistic regression models to assess association between adherence and socioeconomic, health-related, and primary care physician (PCP) characteristics, stratified by age group (< 5, 5‒17, 18‒44, 45‒64, ≥ 65) and sex. Results are adjusted odds ratios with 95% confidence intervals.

RESULTS

There were 152,493 participants. Males had lower odds of being more adherent except among ≥ 65-year-olds (1.03, 95% CI 1.01‒1.05). Compared with the lowest income quintile, those in higher income quintiles had higher odds of being more adherent. The odds mostly increased with increase in income quintile. Those with more contact with their PCP/hospitalization one year prior had higher odds of being more adherent. The odds increased with increased contact among those 18‒44, 45‒64 and ≥ 65 years old. Those who had PCP with more years of practice had higher odds of being more adherent. The odds increased as years of practice increased. These observations were mostly consistent irrespective of sex.

CONCLUSION

Female gender, having higher income, having more contact with the health system, and having an experienced PCP may determine increased adherence to SIV in Manitoba. These findings require attention.

Hemagglutination Inhibition Antibody Titers as Mediators of Influenza Vaccine Efficacy Against Symptomatic Influenza A(H1N1), A(H3N2), and B/Victoria Virus Infections

BACKGROUND

The hemagglutination inhibition antibody (HAI) titer contributes only a part of vaccine-induced protection against influenza virus infections. Using causal mediation analysis, we quantified the proportion of vaccine efficacy mediated by postvaccination HAI titers.

METHODS

We conducted causal mediation analyses using data from a randomized, active-comparator controlled, phase III, trial of an inactivated, split-virion seasonal quadrivalent influenza vaccine in children conducted from October 2010 to December 2011 in 8 countries. Vaccine efficacy was estimated using a weighted Cox proportional hazards model. Estimates were decomposed into the direct and indirect effects mediated by postvaccination HAI titers.

RESULTS

The proportions of vaccine efficacy mediated by postvaccination HAI titers were estimated to be 22% (95% confidence interval, 18%--47%) for influenza A(H1N1), 20% (16%-39%) for influenza A(H3N2), and 37% (26%-85%) for influenza B/Victoria.

CONCLUSIONS

HAI titers partially mediate influenza vaccine efficacy against influenza A(H1N1), A(H3N2), and B/Victoria. Our estimates were lower than in previous studies, possibly reflecting expected heterogeneity in antigenic similarity between vaccine and circulating viruses across seasons.

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.

Influenza A(H3N2) Antibody Responses to Standard-Dose Versus Enhanced Influenza Vaccine Immunogenicity in Older Adults and Prior Season's Vaccine Status

BACKGROUND

Annual influenza vaccination is recommended for older adults but repeated vaccination with standard-dose influenza vaccine has been linked to reduced immunogenicity and effectiveness, especially against A(H3N2) viruses.

METHODS

Community-dwelling Hong Kong adults aged 65-82 years were randomly allocated to receive 2017-2018 standard-dose quadrivalent, MF59-adjuvanted trivalent, high-dose trivalent, and recombinant-HA quadrivalent vaccination. Antibody response to unchanged A(H3N2) vaccine antigen was compared among participants with and without self-reported prior year (2016-2017) standard-dose vaccination.

RESULTS

Mean fold rise (MFR) in antibody titers from day 0 to day 30 by hemagglutination inhibition and virus microneutralization assays were lower among 2017-2018 standard-dose and enhanced vaccine recipients with (range, 1.7-3.0) versus without (range, 4.3-14.3) prior 2016-2017 vaccination. MFR was significantly reduced by about one-half to four-fifths for previously vaccinated recipients of standard-dose and all 3 enhanced vaccines (β range, .21-.48). Among prior-year vaccinated older adults, enhanced vaccines induced higher 1.43 to 2.39-fold geometric mean titers and 1.28 to 1.74-fold MFR versus standard-dose vaccine by microneutralization assay.

CONCLUSIONS

In the context of unchanged A(H3N2) vaccine strain, prior-year vaccination was associated with reduced antibody response among both standard-dose and enhanced influenza vaccine recipients. Enhanced vaccines improved antibody response among older adults with prior-year standard-dose vaccination.

Immune imprinting of SARS-CoV-2 responses: changing first immune impressions

Since the emergence of the ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus and the successful rollout of protective vaccines based on this original strain, SARS-CoV-2 has evolved into several variants, in a classical virus-host arms race typical of RNA viruses, to progressively evade the host immune response. Next-generation bivalent vaccines have been developed with broader protection against emerging variants than the ancestral vaccine. Nonetheless, even these vaccines show lower protection against the latest Omicron variants. Immune printing describes how an immune response to an immunogen is impacted by earlier exposures to a related immunogen. Several lessons about the effect of immune imprinting on responses to SARS-CoV-2 infection and vaccination, including age-associated impacts, can be learned from influenza. Understanding the mechanisms of imprinting of SARS-CoV-2 will be important to inform the design of vaccines that produce broader and more durable protective immune responses to emerging variants.

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.

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.

Systematic review of influenza vaccine effectiveness against laboratory-confirmed influenza among older adults living in aged care facilities

We estimated the effectiveness of influenza vaccines in preventing laboratory-confirmed influenza among older adults in aged care. Electronic database searches were conducted using search terms, and studies were selected as per the selection criteria. Fourteen studies were included for final review. The studies exhibited considerable variation in reported vaccine effectiveness (VE) across different seasons. Among the observational studies, VE ranged from 7.2% to 89.8% against laboratory-confirmed influenza across different vaccines. Randomized clinical trials demonstrated a 17% reduction in infection rates with the adjuvanted trivalent vaccine. The limitations include the small number of included studies conducted in different countries or regions, varied seasons, variations in diagnostic testing methods, a focus on the A/H3N2 strain, and few studies available on the effectiveness of enhanced influenza vaccines in aged care settings. Despite challenges associated with achieving optimal protection, the studies showed the benefits of influenza vaccination in the elderly residents.

Influenza Vaccine Effectiveness in Preventing Laboratory-Confirmed Influenza Cases and Hospitalizations in Navarre, Spain, 2022-2023

We estimated influenza vaccine effectiveness (IVE) in preventing outpatient and hospitalized cases in the 2022-2023 season. A test-negative design included a representative sample of outpatients and all hospitalized patients with influenza-like illness (ILI) from October 2022 to May 2023 in Navarre, Spain. ILI patients were tested by PCR for influenza virus. Influenza vaccination status was compared between confirmed influenza cases and test-negative controls. Among 3321 ILI patients tested, IVE to prevent influenza cases was 34% (95% confidence interval (CI): 16 to 48) overall, 85% (95%CI: 63 to 94) against influenza B, and 28% (95%CI: 3 to 46) against A(H3N2). Among 558 outpatients, 222 (40%) were confirmed for influenza: 55% A(H3N2), 11% A(H1N1), and 31% B. Overall, IVE to prevent outpatient cases was 48% (95%CI: 8 to 70), 88% (95%CI: 3 to 98) against influenza B, and 50% (95%CI: -4 to 76) against A(H3N2). Of 2763 hospitalized patients, 349 (13%) were positive for influenza: 64% A(H3N2), 17% A(H1N1), and 8% B. IVE to prevent hospitalization was 24% (95%CI: -1 to 42) overall, 82% (95%CI: 49 to 93) against influenza B, and 16% (95%CI: -17 to 40) against A(H3N2). No IVE was observed in preventing influenza A(H1N1). IVE was high to prevent influenza B, moderate against A(H3N2) and null against A(H1N1). A lower proportion of influenza B cases may explain the smaller IVE in hospitalized patients than in outpatients. The null IVE against A(H1N1) was consistent with the observed antigenic drift and supports the new composition of the 2023-2024 influenza vaccine.

Rational design of a booster vaccine against COVID-19 based on antigenic distance

Current COVID-19 vaccines are highly effective against symptomatic disease, but repeated booster doses using vaccines based on the ancestral strain offer limited additional protection against SARS-CoV-2 variants of concern (VOCs). To address this, we used antigenic distance to in silico select optimized booster vaccine seed strains effective against both current and future VOCs. Our model suggests that a SARS-CoV-1-based booster vaccine has the potential to cover a broader range of VOCs. Candidate vaccines including the spike protein from ancestral SARS-CoV-2, Delta, Omicron (BA.1), SARS-CoV-1, or MERS-CoV were experimentally evaluated in mice following two doses of the BNT162b2 vaccine. The SARS-CoV-1-based booster vaccine outperformed other candidates in terms of neutralizing antibody breadth and duration, as well as protective activity against Omicron (BA.2) challenge. This study suggests a unique strategy for selecting booster vaccines based on antigenic distance, which may be useful in designing future booster vaccines as new SARS-CoV-2 variants emerge.

Summary of the National Advisory Committee on Immunization (NACI) Statement-Recommendation on Repeated Seasonal Influenza Vaccination

Background

Influenza vaccination is recommended annually; however, some studies have raised questions regarding whether repeated influenza vaccine administration may have unintended negative consequences for seasonal protection.

Methods

The National Advisory Committee on Immunization (NACI) Influenza Working Group undertook an overview of systematic reviews on the effects of repeated influenza vaccination on vaccine effectiveness, efficacy, and immunogenicity. A systematic assessment of programmatic factors was conducted according to established NACI methods. The NACI evidence-based process was used to critically appraise the available evidence and to review recommendations.

Results

The evidence base consisted of four eligible systematic reviews/meta-analyses. Repeated vaccination, including the current season, was consistently more effective than no vaccination in the current season. The evidence showed no significant difference or predictable trend in vaccine efficacy or effectiveness between vaccinations in two consecutive seasons compared to vaccination in the current season only.

Conclusion

Overall, NACI concluded that there is evidence to recommend annual influenza vaccination, irrespective of whether an individual received the seasonal influenza vaccine in previous seasons. It is neither currently feasible nor warranted to modify existing annual influenza vaccination programs to account for potential negative or positive interference. NACI continues to strongly recommend that seasonal influenza vaccine should be offered annually to everyone six months of age and older who does not have contraindications to the vaccine, irrespective of previous seasons' influenza vaccination status.

Does repeated influenza vaccination attenuate effectiveness? A systematic review and meta-analysis

BACKGROUND

Influenza vaccines require annual readministration; however, several reports have suggested that repeated vaccination might attenuate the vaccine's effectiveness. We aimed to estimate the reduction in vaccine effectiveness associated with repeated influenza vaccination.

METHODS

In this systematic review and meta-analysis, we searched MEDLINE, EMBASE, and CINAHL Complete databases for articles published from Jan 1, 2016, to June 13, 2022, and Web of Science for studies published from database inception to June 13, 2022. For studies published before Jan 1, 2016, we consulted published systematic reviews. Two reviewers (EJ-G and EJR) independently screened, extracted data using a data collection form, assessed studies' risk of bias using the Risk Of Bias In Non-Randomized Studies of Interventions (ROBINS-I) and evaluated the weight of evidence by Grading of Recommendations Assessment, Development, and Evaluation (GRADE). We included observational studies and randomised controlled trials that reported vaccine effectiveness against influenza A(H1N1)pdm09, influenza A(H3N2), or influenza B using four vaccination groups: current season; previous season; current and previous seasons; and neither season (reference). For each study, we calculated the absolute difference in vaccine effectiveness (ΔVE) for current season only and previous season only versus current and previous season vaccination to estimate attenuation associated with repeated vaccination. Pooled vaccine effectiveness and ∆VE were calculated by season, age group, and overall. This study is registered with PROSPERO, CRD42021260242.

FINDINGS

We identified 4979 publications, selected 681 for full review, and included 83 in the systematic review and 41 in meta-analyses. ΔVE for vaccination in both seasons compared with the current season was -9% (95% CI -16 to -1, I2=0%; low certainty) for influenza A(H1N1)pdm09, -18% (-26 to -11, I2=7%; low certainty) for influenza A(H3N2), and -7% (-14 to 0, I2=0%; low certainty) for influenza B, indicating lower protection with consecutive vaccination. However, for all types, A subtypes and B lineages, vaccination in both seasons afforded better protection than not being vaccinated.

INTERPRETATION

Our estimates suggest that, although vaccination in the previous year attenuates vaccine effectiveness, vaccination in two consecutive years provides better protection than does no vaccination. The estimated effects of vaccination in the previous year are concerning and warrant additional investigation, but are not consistent or severe enough to support an alternative vaccination regimen at this time.

FUNDING

WHO and the US National Institutes of Health.

[Effectiveness of the influenza vaccine in the prevention of influenza in people over 65 years of age]

INTRODUCTION

Influenza is one of the diseases with the greatest epidemiological impact and of maximum relevance in the management of health services. The flu vaccine can have great variability each season, so our objective was to find out the effectiveness of the flu vaccine for the 2017/2018 season for the prevention of severe cases of flu in people over 65 years of age in a 385-bed acute general hospital.

MATERIAL AND METHOD

Study of cases and controls. All hospitalized patients with laboratory-confirmed influenza older than 65 years during the 2017/2018 season were included. Those who met the criteria for a severe case of influenza were considered cases. Those who did not meet the severity criteria were considered controls. Factors associated with the development of severe influenza were calculated.

RESULTS

The median age was 68 years (SD 91.87). The attack rate was 0.23 per hundred inhabitants and the vaccine effectiveness was 38%. The vaccinated and unvaccinated groups were different in terms of age (p < 0.0481). Vaccination status against severe influenza was found to be an independent protective factor (OR = 0.840; 0.746-0.913).

CONCLUSIONS

The effectiveness of influenza vaccination provided greater protection against infection and reduced the severity of influenza in older hospitalized patients. These findings should be taken into account to improve vaccination strategies and achieve better vaccination coverage in the population at risk.

Trivalent inactivated influenza vaccine response and immunogenicity assessment after one week and three months in repeatedly vaccinated adults

BACKGROUND

The influenza vaccine administrated every year is a recommended infection control procedure for individuals above the age of six months. However, the effectiveness of repeated annual vaccination is still an active research topic. Therefore, we investigated the vaccine immunogenicity in two independent groups: previously vaccinated versus non-vaccinated individuals at three time points; prior vaccination, one week and three months post vaccination. The assessment enabled us to evaluate the elicited immune responses and the durability of the induced protection in both groups.

RESEARCH DESIGN AND METHODS

A research study was conducted to assess the immunogenicity of a single dose of Trivalent Inactivated Influenza Vaccine (A/H1N1, A/H3N2, and B) in 278 healthy adults aged between 32 and 66 years. Almost half of the participants, 140 (50·36%), received influenza vaccination at least once precursor to past influenza seasons. One blood sample was taken prior to vaccination for complete blood analysis and baseline immunogenicity assessment. The selected study participants received a single vaccine dose on the first day, and then followed up for three months. Two blood samples were taken after one week and three months post vaccination, respectively, for vaccine immunogenicity assessment.

RESULTS

Before vaccination, the seroprotection, defined as a hemagglutination-inhibiting titer of =>1:40, was detected for the three vaccine virus strains in 20 previously vaccinated participants (14·29%) [8·95%, 21·2%]. We compared the overall vaccine response for the three virus strains using a normalized response score calculated from linearly transformed titer measurements; the score before vaccination was 84% higher in the previously vaccinated group and the mean difference between the two groups was statistically significant. Three months post-vaccination, we didn't find a significant difference in vaccine responses; the number of fully seroprotected individuals became 48 (34·29%) [26·48%, 42·77%] in the previously vaccinated group and 59 (42·75%) [34·37%, 51·45%] in the non-vaccinated group. The calculated response score was almost equal in both groups and the mean difference was no longer statistically significant.

CONCLUSION

Our findings suggest that a single dose of influenza vaccine is equally protective after three months for annually vaccinated adults and first-time vaccine receivers.

The impact of repeated vaccination on relative influenza vaccine effectiveness among vaccinated adults in the United Kingdom

Annual seasonal influenza vaccination is recommended for individuals at high risk of developing post-infection complications in many locations. However, reduced vaccine immunogenicity and effectiveness have been observed among repeat vaccinees in some influenza seasons. We investigated the impact of repeated influenza vaccination on relative vaccine effectiveness (VE) among individuals who were recommended for influenza vaccination in the United Kingdom with a retrospective cohort study using primary healthcare data from the Clinical Practice Research Datalink, a primary care database in the United Kingdom. Relative VE was estimated against general practitioner-diagnosed influenza-like illnesses (GP-ILI) and medically attended acute respiratory illnesses (MAARI) among participants who have been repeatedly vaccinated compared with first-time vaccinees using proportional hazards models. Relative VE against MAARI may be reduced for individuals above 65 years old who were vaccinated in the current and previous influenza seasons for some influenza seasons. However, these findings were not conclusive as we could not exclude the possibility of residual confounding in our dataset. The use of routinely collected data from electronic health records to examine the effects of repeated vaccination needs to be complemented with sufficient efforts to include negative control outcomes to rule out residual confounding.

[Effectiveness of influenza vaccine in preventing severe influenza]

INTRODUCTION

Influenza is one of the diseases with the greatest epidemiological impact and the greatest relevance in the management of health services. The flu vaccine can have great variability each season, so our objective was to know the effectiveness of the flu vaccine for the 2017/2018 season for the prevention of severe cases of flu in a general acute hospital in 385 beds.

MATERIAL AND METHOD

Case control study. All hospitalized patients with laboratory confirmed influenza during the 2017/2018 season were included. Those who met the criteria for a severe case of influenza were considered cases. Those that did not meet the severity criteria were considered controls. The factors associated with the development of severe influenza were calculated.

RESULTS

The effectiveness adjusted by age group and comorbidity was 60.7% (20.5-80.5). The vaccinated and unvaccinated groups were different in terms of age (P<.0381). The highest proportion of cases were concentrated in those over 65 years of age (45.5%). Vaccination status against severe influenza was found to be an independent protective factor (OR=.746; .694-.831).

CONCLUSIONS

The effectiveness of influenza vaccination provided greater protection against infection and reduced the severity of influenza in hospitalized patients. These findings should be considered to improve vaccination strategies and achieve better vaccination coverage in the population at risk.

Effect of Text Message Communication on Patient Presentation for an Influenza Vaccination in a Community-Based Pharmacy Setting

Background

Community pharmacies use text message communications for information regarding approaching refills and fill status. Patients can also be notified regarding annual influenza vaccine availability and schedule an appointment for the vaccine.

Objectives

This study aimed to evaluate whether text message communications affected patient presentation and resulted in a percent increase of patients receiving an influenza vaccine compared with previous vaccine season and to determine whether additional vaccines are administered upon presentation.

Methods

Ambidirectional study retrospectively analyzed the impact, nationally, of a new text message communication on influenza vaccinations at a large community pharmacy chain and prospectively surveyed patients receiving an influenza vaccine at 2 geographically similar pharmacies of the chain in Southwest Virginia. Text message communications regarding vaccine and appointment availability were sent to patients at the age of 18 years and older who opted in to text message communications and received an influenza vaccine with the chain during the 2019-2020 influenza season. Vaccine data from consecutive seasons were compared. Eligible patients in Southwest Virginia were surveyed about how they were informed about availability, previous intent to receive an influenza vaccine, applicability to other vaccines, and effect of the coronavirus disease 2019 pandemic on vaccination. Results were analyzed using bivariate and multivariate analyses.

Results

Nationally, influenza vaccines administered increased by 17.45% in patients who permitted text message communication and overall by 13.22% after implementation. Decreases in co-administered pneumococcal vaccines and tetanus, diphtheria, and pertussis vaccines and an increase in co-administered zoster vaccines were observed. A total of 111 patients were surveyed; 4% presented owing to text message communication. A majority were intent on receiving the vaccine before being notified and reported that the pandemic did not affect presentation. Notably, 45.05% of patients were likely to receive routine vaccines if notified by text message.

Conclusion

Text message communications are another viable way to increase vaccinations, but further studies should be conducted outside of a pandemic setting.

Effectiveness of influenza vaccination in preventing influenza in primary care, Navarre, Spain, 2021/22

Compared with individuals unvaccinated in the current and three previous influenza seasons, in 2021/22, influenza vaccine effectiveness at primary care level was 37% (95% CI: 16 to 52) for current season vaccination, regardless of previous doses, and 35% (95% CI: −3 to 45) for only previous seasons vaccination. Against influenza A(H3N2), estimates were 39% (95% CI: 16 to 55) and 24% (95% CI: −8 to 47) suggesting moderate effectiveness of current season vaccination and possible remaining effect of prior vaccinations.

Influenza Vaccination and Risk of Stroke in Women With Chronic Obstructive Pulmonary Disease: A Nationwide, Population-Based, Propensity-Matched Cohort Study

Backgrounds

The risk of stroke is higher among patients with chronic obstructive pulmonary disease (COPD) than among the healthy population. Moreover, women generally have worse long-term stroke outcomes than men.

Methods

The data of 6681 women with COPD (aged ≥ 65 years) registered in Taiwan’s National Health Insurance Research Database were retrospectively analyzed from January 1, 2001 to December 31, 2011. After 1:1 propensity score matching, the patients were divided into vaccinated and unvaccinated groups.

Results

In total, 5102 women were enrolled. The vaccinated group had a significantly lower risk of total, hemorrhagic, and ischemic stroke than the unvaccinated group (adjusted hazard ratio [aHR]: 0.60, 95% confidence interval [CI]: 0.54–0.67; aHR: 0.59, 95% CI: 0.43–0.83; and aHR: 0.59, 95% CI: 0.52–0.68, respectively). A lower risk of stroke was observed among the women aged 65–74 and ≥75 years, and the association was dose-dependent in all types of stroke (aHR: 1.08, 95% CI: 0.92–1.26; aHR: 0.70, 95% CI: 0.60–0.82; and aHR: 0.32, 95% CI: 0.26–0.38 for those vaccinated 1, 2 to 3, and ≥4 times, respectively, during the follow-up period). Women with a CHA2DS2-VASc score (conditions and characteristics included congestive heart failure, hypertension, diabetes, stroke, vascular disease, age, and sex) of 2–3 and ≥4 had a significantly lower risk of ischemic stroke while receiving more vaccinations. A smaller significant lower risk of hemorrhagic stroke after more than 4 times of vaccination was noted in the women with a CHA2DS2-VASc score of ≥4. Both interrupted and non-interrupted vaccination was associated with lower risk of stroke occurrence.

Conclusion

Influenza vaccination is associated with a lower risk of total, hemorrhagic, and ischemic stroke among women with COPD, and the association is dose-dependent. However, the findings may be limited by unmeasurable confounders. Further investigations on this subject are warranted.

Impact of prior infection and repeated vaccination on post-vaccination antibody titers of the influenza A(H1N1)pdm09 strain in Taiwan schoolchildren: Implications for public health

BACKGROUND

The objective of this study was to evaluate the effects of prior-infection and repeated vaccination on post-vaccination antibody titers.

METHODS

A(H1N1)pdm09 strain was included in 2009 pandemic monovalent, 2010-2011, and 2011-2012 trivalent influenza vaccines (MIVpdm09, TIV10/11, TIV11/12) in Taiwan. During the 2011-2012 influenza season, we conducted a prospective sero-epidemiological cohort study among schoolchildren from grades 1 - 6 in the two elementary schools in Taipei with documented A(H1N1)pdm09 vaccination records since 2009. Serum samples were collected at pre-vaccination, 1-month, and 4-months post-vaccination (T1, T2, T3). Anti-A(H1N1)pdm09 hemagglutination inhibition titers (HI-Ab-titers) were examined. We also investigated the impact of four vaccination histories [(1) no previous vaccination (None), (2) vaccinated in 2009-2010 season (09v), (3) vaccinated in 2010-2011 season (10v), and (4) vaccinated consecutively in 2009-2010 and 2010-2011 seasons (09v + 10v)] and pre-vaccination HI-Ab levels on post-vaccination HI-Ab responses as well as adjusted vaccine effectiveness (aVE) against serologically-defined infection from T2 to T3.

RESULTS

TIV11/12 had zero serious adverse events reported. A(H1N1)pdm09 strain in TIV11/12 elicited seroprotective Ab-titers in 98% of children and showed promising protection (aVE: 70.3% [95% confidence interval (CI): 51.0-82.1%]). Previously unvaccinated but infected children had a 3.96 times higher T2 geometric mean titer (T2-GMT) of HI-Ab than those naïve to A(H1N1)pdm09 (GMT [95% CI]: 1039.7[585.3-1845.9] vs. 262.5[65.9-1045], p = 0.046). Previously vaccinated children with seroprotective T1-Ab-titers had a higher T2-GMT and a greater aVE than those with non-seroprotective T1-Ab-titers. Repeatedly vaccinated children had lower T2-GMT than those receiving primary doses of TIV11/12. However, after controlling prior infection and T1-Ab-titers, differences in T2-GMT among the four vaccination histories became insignificant (p = 0.16).

CONCLUSION

This study supports the implementation of annual mass-vaccination with A(H1N1)pdm09 in schoolchildren for three consecutive influenza seasons when vaccine and circulating strains were well matched, and found that prior infection and pre-vaccination HI-Ab levels positively impacted post-vaccination HI-Ab responses.

Estimation of Relative Vaccine Effectiveness in Influenza: A Systematic Review of Methodology

BACKGROUND

When new vaccine components or platforms are developed, they will typically need to demonstrate noninferiority or superiority over existing products, resulting in the assessment of relative vaccine effectiveness (rVE). This review aims to identify how rVE evaluation is being performed in studies of influenza to inform a more standardized approach.

METHODS

We conducted a systematic search on PubMed, Google Scholar, and Web of Science for studies reporting rVE comparing vaccine components, dose, or vaccination schedules. We screened titles, abstracts, full texts, and references to identify relevant articles. We extracted information on the study design, relative comparison made, and the definition and statistical approach used to estimate rVE in each study.

RESULTS

We identified 63 articles assessing rVE in influenza virus. Studies compared multiple vaccine components (n = 38), two or more doses of the same vaccine (n = 17), or vaccination timing or history (n = 9). One study compared a range of vaccine components and doses. Nearly two-thirds of all studies controlled for age, and nearly half for comorbidities, region, and sex. Assessment of 12 studies presenting both absolute and relative effect estimates suggested proportionality in the effects, resulting in implications for the interpretation of rVE effects.

CONCLUSIONS

Approaches to rVE evaluation in practice is highly varied, with improvements in reporting required in many cases. Extensive consideration of methodologic issues relating to rVE is needed, including the stability of estimates and the impact of confounding structure on the validity of rVE estimates.

Chronic obstructive pulmonary disease and influenza vaccination effect in preventing outpatient and inpatient influenza cases

Evidence of influenza vaccine effectiveness in preventing confirmed influenza among persons diagnosed with chronic obstructive pulmonary disease (COPD) is scarce. We assessed the average effect of influenza vaccination in the current and prior seasons in preventing laboratory-confirmed influenza in COPD patients. We carried out a pooled test-negative case–control design in COPD patients hospitalized or presented to primary healthcare centres with influenza-like illness who were tested for influenza in 2015/2016 to 2019/2020 seasons in Navarre, Spain. Influenza vaccination status in the current and 5 prior seasons was compared between confirmed-influenza cases and test-negative controls. Vaccination effect was compared between target patients for vaccination with and without COPD. Out of 1761 COPD patients tested, 542 (31%) were confirmed for influenza and 1219 were test-negative controls. Average effect for current-season vaccination in preventing influenza was 40% (95% CI 20–54%), and for vaccination in prior seasons only was 24% (95% CI –10 to 47%). Point estimates seemed higher in preventing outpatient cases (60% and 58%, respectively) than inpatient cases (37% and 19%, respectively), but differences were no statistically significant. Influenza vaccination effect was similar in target population with and without COPD (p = 0.339). Influenza vaccination coverage in control patients with COPD was 68.3%. A 13.7% of the influenza cases in patients with COPD could be prevented by extending the influenza vaccine coverage. Average effect of current-season influenza vaccination was moderate to prevent influenza in COPD persons. The increase of influenza vaccination coverage can still prevent COPD exacerbations.

Effectiveness of seasonal influenza vaccine in adult Japanese workers, 2017-2020

BACKGROUND

Previous studies have not estimated vaccine effectiveness (VE) against influenza in the working-age Japanese population. In this study, we determined VE in adult workers at a Japanese company.

METHODS

We estimated VE based on self-reported data regarding influenza infections and vaccinations in employees of an auto parts manufacturing company during three influenza seasons from 2017 to 2020. VE was estimated as 100% × [1 - odds ratio (the ratio of the odds of being diagnosed with influenza among enrollees with and without influenza vaccination)]. Odds ratios were estimated using logistic regression.

RESULTS

We included 11,347 worker records [3,592 (2017-18), 3,663 (2018-19), and 4,092 (2019-20)] from employees who had worked with the company throughout each influenza season. The adjusted VE was moderate and significant in the 2019-20 season (VE = 53%; 95% confidence interval [CI] = 30% to 69%) but low or negative and non-significant during the 2017-18 (VE = 28%; 95% CI = -5% to 50%) and 2018-19 (VE = -11%; 95% CI =  - 42% to 14%) seasons.

CONCLUSIONS

Influenza vaccines were moderately effective during the 2019-20 season but showed low or negative effectiveness during the 2017-18 and 2018-19 seasons. Self-reports from worker records can successfully help determine VE against influenza.

Influence of Prior Influenza Vaccination on Current Influenza Vaccine Effectiveness in Children Aged 1 to 5 Years

BACKGROUND

Although annual influenza vaccination is an important strategy used to prevent influenza-related morbidity and mortality, some studies have reported the negative influence of prior vaccination on vaccine effectiveness (VE) for current seasons. Currently, the influence of prior vaccination is not conclusive, especially in children.

METHODS

We evaluated the association between current-season VE and prior season vaccination using a test-negative design in children aged 1-5 years presenting at nine outpatient clinics in Japan during the 2016/17 and 2017/18 influenza seasons. Children with influenza-like illness were enrolled prospectively and tested for influenza using real-time RT-PCR. Their recent vaccination history was categorized into six groups according to current vaccination doses (0/1/2) and prior vaccination status (unvaccinated = 0 doses/vaccinated = 1 dose or 2 doses): (1) 0 doses in the current season and unvaccinated in prior seasons (reference group); (2) 0 doses in the current season and vaccinated in a prior season; (3) 1 dose in the current season and unvaccinated in a prior season; (4) 1 dose in the current season and vaccinated in a prior season; (5) 2 doses in the current season and unvaccinated in a prior season, and (6) 2 doses in the current season and vaccinated in a prior season.

RESULTS

A total of 799 cases and 1196 controls were analyzed. The median age of the subjects was 3 years, and the proportion of males was 54%. Overall, the vaccination rates (any vaccination in the current season) in the cases and controls were 36% and 53%, respectively. The VEs of the groups were: (2) 29% (95% confidence interval: -25% to 59%); (3) 53% (6% to 76%); (4) 70% (45% to 83%); (5) 56% (32% to 72%), and (6) 61% (42% to 73%). The one- and two-dose VEs of the current season were significant regardless of prior vaccination status. The results did not differ when stratified by influenza subtype/lineage.

CONCLUSION

Prior vaccination did not attenuate the current-season VE in children aged 1 to 5 years, supporting the annual vaccination strategy.

Repeated influenza vaccination and hospitalization outcomes among older patients with cardiovascular or respiratory diseases

Influenza vaccination in a single season protects against hospitalization outcomes among older adults hospitalized for cardiovascular or respiratory diseases, but the effectiveness of repeated influenza vaccination is less clear. Four hospitalization outcomes (in-hospital death, re-admission, length of stay, and direct medical costs) were extracted from the Beijing Urban Employee Basic Medical Insurance database in 2015-2016 for adults aged ≥60 years hospitalized for cardiovascular or respiratory diseases. Vaccination status during three influenza seasons (2013/2014-2015/2016) was ascertained through linkages to the Beijing Elderly Influenza Vaccination database. The summer months (June-August) were used as a reference period to control unmeasured confounders during the influenza season. There were 99,135 periods of observation in the analysis, with 8.3% participants receiving influenza vaccination in all three seasons. After adjusting for confounders, influenza vaccination in all three seasons was associated with a lower risk of re-admission among patients with cardiovascular diseases (odds ratio 0.71 [95% CI 0.53-0.96]) and a lower risk of death among patients with respiratory diseases (0.68 [0.46-0.98]) compared with those unvaccinated in any season. Among patients with cardiovascular diseases, influenza vaccination in all three seasons was also associated with a non-significant lower risk of death (0.66 [0.44-1.03]) in addition to shorter hospital stays and lower direct medical costs. When stratified by history of vaccination, the effectiveness of current season vaccination was similar among patients with cardiovascular or respiratory diseases (p-value for heterogeneity all >0.05). Repeated influenza vaccination protected against hospitalization outcomes among older adults with cardiovascular or respiratory diseases.

Risk factors associated with respiratory infectious disease-related presenteeism: a rapid review

BACKGROUND

Workplace transmission is a significant contributor to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreaks. Previous studies have found that infectious illness presenteeism could contribute to outbreaks in occupational settings and identified multiple occupational and organisational risk factors. Amid the COVID-19 pandemic, it is imperative to investigate presenteeism particularly in relation to respiratory infectious disease (RID). Hence, this rapid review aims to determine the prevalence of RID-related presenteeism, including COVID-19, and examines the reported reasons and associated risk factors.

METHODS

The review followed a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) search approach and focused on studies published in English and Chinese. Database searches included MEDLINE, EMBASE, Web of Science, China Knowledge Resource Integrated Database (CNKI) and preprint databases MedRxiv and BioRxiv.

RESULTS

The search yielded 54 studies, of which four investigated COVID-19-related presenteeism. Prevalence of work presenteeism ranged from 14.1 to 55% for confirmed RID, and 6.6 to 100% for those working with suspected or subclinical RID. The included studies demonstrated that RID-related presenteeism is associated with occupation, sick pay policy, age, gender, health behaviour and perception, vaccination, peer pressure and organisational factors such as presenteeism culture.

CONCLUSIONS

This review demonstrates that presenteeism or non-adherence to isolation guidance is a real concern and can contribute to workplace transmissions and outbreaks. Policies which would support workers financially and improve productivity, should include a range of effective non-pharmaceutical inventions such as workplace testing, promoting occupational health services, reviewing pay and bonus schemes and clear messaging to encourage workers to stay at home when ill. Future research should focus on the more vulnerable and precarious occupational groups, and their inter-relationships, to develop comprehensive intervention programs to reduce RID-related presenteeism.

Influenza A-Associated In-Hospital Mortality in Very Older People: Does Inflammation Also Play a Role?

BACKGROUND

The aim of the study was to analyze the clinical manifestations and outcome of the oldest old (people aged ≥85 years) who were admitted to the hospital with a confirmed influenza A virus infection in comparison with younger patients and to assess the role of inflammation in the outcome of influenza infection in this population.

METHODS

This is an observational prospective study including all adult patients with influenza A virus infection hospitalized in a tertiary teaching hospital in Madrid, in 2 consecutive influenza seasons (2016-17 and 2017-18).

RESULTS

Five hundred nine hospitalized patients with influenza A infection were included, of whom 117 (23%) were older than 85 years (median age: 89.3 ± 3.2). We compared the clinical characteristics and outcome with those of the rest of the population (median age: 72.8 ± 15.7). Overall, mortality was higher in older patients (10% vs. 4%; p = 0.03) with no differences in clinical presentation. Patients older than 85 years who ultimately died (12 out of 117) showed increased systemic inflammation expressed by higher levels of C-reactive protein (CRP) and ferritin compared to survivors who were discharged (odds ratio [OR] of CRP >20 mg/dL: 5.16, 95% confidence interval [CI]: 1.29-20.57, and OR of ferritin >500 mg: 4.3, 95% CI: 1.04-17.35).

CONCLUSIONS

Patients aged 85 and older with influenza A virus infection presented a higher in-hospital mortality than younger subjects. CRP and ferritin levels were higher in the oldest old who died, suggesting that inflammation could play a key role in the outcome of this subset of patients.

Poor Vaccine Effectiveness against Influenza B-Related Severe Acute Respiratory Infection in a Temperate North Indian State (2019-2020): A Call for Further Data for Possible Vaccines with Closer Match

Background: Influenza vaccine uptake in India is poor, and scant data exist regarding the effectiveness of influenza vaccine against hospitalization. Methods: From October 2019 to March 2020, vaccination status of 1219 patients (males n = 571, aged 5–107 years; median, 50 years) hospitalized with severe acute respiratory illness (SARI) was assessed. The patients were tested for influenza viruses and their subtypes by RT PCR. Sequencing of the HA gene was performed. Vaccine effectiveness (VE) against influenza subtypes was estimated by the test negative design. Results: A total of 336 (27.5%) patients were influenza-positive, with influenza B/Victoria accounting for 49.7% (n = 167), followed by influenza A/H1N1 (47.6%; n = 155) and influenza A/H3N2 (4.4%; n = 15). About 6.8% and 8.6% of the influenza-positive and influenza-negative patients, respectively, had been vaccinated. Adjusted VE for any influenza strain was 13% (95% CI −42 to 47), which for influenza B was 0%. HA sequencing revealed that influenza B samples mainly belonged to subclade V1A.3/133R with deletion of residues 163–165, as against the 2-aa deletion in influenza B/Colorado/06/2017 strain, contained in the vaccine. VE for influenza A/H1N1 was 55%. Conclusions: Poor VE due to a genetic mismatch between the circulating strain and the vaccine strain calls for efforts to reduce the mismatch.

Simple models to include influenza vaccination history when evaluating the effect of influenza vaccination

Background

Most reports of influenza vaccine effectiveness consider current-season vaccination only.

Aim

We evaluated a method to estimate the effect of influenza vaccinations (EIV) considering vaccination history.

Methods

We used a test-negative design with well-documented vaccination history to evaluate the average EIV over eight influenza seasons (2011/12–2018/19; n = 10,356). Modifying effect was considered as difference in effects of vaccination in current and previous seasons and current-season vaccination only. We also explored differences between current-season estimates excluding from the reference category people vaccinated in any of the five previous seasons and estimates without this exclusion or only for one or three previous seasons.

Results

The EIV was 50%, 45% and 38% in people vaccinated in the current season who had previously received none, one to two and three to five doses, respectively, and it was 30% and 43% for one to two and three to five prior doses only. Vaccination in at least three previous seasons reduced the effect of current-season vaccination by 12 percentage points overall, 31 among outpatients, 22 in 9–65 year-olds, and 23 against influenza B. Including people vaccinated in previous seasons only in the unvaccinated category underestimated EIV by 9 percentage points on average (31% vs 40%). Estimates considering vaccination of three or five previous seasons were similar.

Conclusions

Vaccine effectiveness studies should consider influenza vaccination in previous seasons, as it can retain effect and is often an effect modifier. Vaccination status in three categories (current season, previous seasons only, unvaccinated) reflects the whole EIV.

Effects of Prior Season Vaccination on Current Season Vaccine Effectiveness in the United States Flu Vaccine Effectiveness Network, 2012-2013 Through 2017-2018

BACKGROUND

We compared effects of prior vaccination and added or lost protection from current season vaccination among those previously vaccinated.

METHODS

Our analysis included data from the US Flu Vaccine Effectiveness Network among participants ≥9 years old with acute respiratory illness from 2012-2013 through 2017-2018. Vaccine protection was estimated using multivariate logistic regression with an interaction term for effect of prior season vaccination on current season vaccine effectiveness. Models were adjusted for age, calendar time, high-risk status, site, and season for combined estimates. We estimated protection by combinations of current and prior vaccination compared to unvaccinated in both seasons or current vaccination among prior vaccinated.

RESULTS

A total of 31 819 participants were included. Vaccine protection against any influenza averaged 42% (95% confidence interval [CI], 38%-47%) among those vaccinated only the current season, 37% (95% CI, 33-40) among those vaccinated both seasons, and 26% (95% CI, 18%-32%) among those vaccinated only the prior season, compared with participants vaccinated neither season. Current season vaccination reduced the odds of any influenza among patients unvaccinated the prior season by 42% (95% CI, 37%-46%), including 57%, 27%, and 55% against A(H1N1), A(H3N2), and influenza B, respectively. Among participants vaccinated the prior season, current season vaccination further reduced the odds of any influenza by 15% (95% CI, 7%-23%), including 29% against A(H1N1) and 26% against B viruses, but not against A(H3N2).

CONCLUSIONS

Our findings support Advisory Committee on Immunization Practices recommendations for annual influenza vaccination. Benefits of current season vaccination varied among participants with and without prior season vaccination, by virus type/subtype and season.

Effect of influenza vaccination in patients with asthma

BACKGROUND:

Although annual influenza vaccination is recommended for persons with asthma, its effectiveness in this patient population is not well described. We evaluated the effect of influenza vaccination in the current and previous seasons in preventing influenza among people with asthma.

METHODS:

Using population health data from the Navarre region of Spain for the 2015/16 to 2019/20 influenza seasons, we conducted a test-negative case–control study to assess the effect of influenza vaccination in the current and 5 previous seasons. From patients presenting to hospitals and primary health care centres with influenza-like illness who underwent testing for influenza, we estimated the effects of influenza vaccination among patients with asthma overall and between those presenting as inpatients or outpatients, as well as between patients with and without asthma.

RESULTS:

Of 1032 patients who had asthma and were tested, we confirmed that 421 had influenza and the remaining 611 were test-negative controls. We found that the average effect of influenza vaccination was 43% (adjusted odds ratio [OR] 0.57, 95% confidence interval [CI] 0.40 to 0.80) for current-season vaccination regardless of previous doses, and 38% (adjusted OR 0.62, 95% CI 0.39 to 0.96) for vaccination in previous seasons only. Effects were similar for outpatients and inpatients. Among patients with asthma and confirmed influenza, current-season vaccination did not reduce the odds of hospital admission (adjusted OR 1.05, 95% CI 0.51 to 2.18). Influenza vaccination effects were similar for patients with and without asthma.

INTERPRETATION:

We estimated that, on average, current or previous influenza vaccination of people with asthma prevented almost half of influenza cases. These results support recommendations that people with asthma receive influenza vaccination.

Effect of Influenza Vaccination in Preventing Laboratory-Confirmed Influenza Hospitalization in Patients With Diabetes Mellitus

BACKGROUND

People with diabetes are at high risk of severe influenza complications. The influenza vaccination effect among diabetic patients remains inconclusive. We estimated the average effect of influenza vaccination status in the current and prior seasons in preventing laboratory-confirmed influenza hospitalization in diabetic patients.

METHODS

Patients attended in hospitals and primary healthcare centers with influenza-like illness were tested for influenza from the 2013-2014 to 2018-2019 seasons in Navarre, Spain. A test-negative case-control design in diabetic inpatients compared the influenza vaccination status in the current and 5 prior seasons between laboratory-confirmed influenza cases and negative controls. Vaccination status of influenza-confirmed cases was compared between diabetic inpatients and outpatients. Influenza vaccination effect was compared between diabetic patients and older (≥ 60 years) or chronic nondiabetic patients.

RESULTS

Of 1670 diabetic inpatients tested, 569 (34%) were confirmed for influenza and 1101 were test-negative controls. The average effect in preventing influenza hospitalization was 46% (95% confidence interval [CI], 28%-59%) for current-season vaccination and 44% (95% CI, 20%-61%) for vaccination in prior seasons only in comparison to unvaccinated patients in the current and prior seasons. Among diabetic patients with confirmed influenza, current-season vaccination reduced the probability of hospitalization (adjusted odds ratio, 0.35; 95% CI, .15-.79). In diabetic patients, vaccination effect against influenza hospitalizations was not inferior to that in older or chronic nondiabetic patients.

CONCLUSIONS

On average, influenza vaccination of diabetic population reduced by around half the risk of influenza hospitalization. Vaccination in prior seasons maintained a notable protective effect. These results reinforce the recommendation of influenza vaccination for diabetic patients.

Estimates of Inactivated Influenza Vaccine Effectiveness Among Children in Senegal: Results From 2 Consecutive Cluster-Randomized Controlled Trials in 2010 and 2011

BACKGROUND

We report results of years 2 and 3 of consecutive cluster-randomized controlled trials of trivalent inactivated influenza vaccine (IIV3) in Senegal.

METHODS

We cluster-randomized (1:1) 20 villages to annual vaccination with IIV3 or inactivated poliovirus vaccine (IPV) of age-eligible residents (6 months-10 years). The primary outcome was total vaccine effectiveness against laboratory-confirmed influenza illness (LCI) among age-eligible children (modified intention-to-treat population [mITT]). Secondary outcomes were indirect (herd protection) and population (overall community) vaccine effectiveness.

RESULTS

We vaccinated 74% of 12 408 age-eligible children in year 2 (June 2010-April 11) and 74% of 11 988 age-eligible children in year 3 (April 2011-December 2011) with study vaccines. Annual cumulative incidence of LCI was 4.7 (year 2) and 4.2 (year 3) per 100 mITT child vaccinees of IPV villages. In year 2, IIV3 matched circulating influenza strains. The total effectiveness was 52.8% (95% confidence interval [CI], 32.3-67.0), and the population effectiveness was 36.0% (95% CI, 10.2-54.4) against LCI caused by any influenza strain. The indirect effectiveness against LCI by A/H3N2 was 56.4% (95% CI, 39.0-68.9). In year 3, 74% of influenza detections were vaccine-mismatched to circulating B/Yamagata and 24% were vaccine-matched to circulating A/H3N2. The year 3 total effectiveness against LCI was -14.5% (95% CI, -81.2-27.6). Vaccine effectiveness varied by type/subtype of influenza in both years.

CONCLUSIONS

IIV3 was variably effective against influenza illness in Senegalese children, with total and indirect vaccine effectiveness present during the year when all circulating strains matched the IIV3 formulation.

CLINICAL TRIALS REGISTRATION

NCT00893906.

Repeated influenza vaccination provides cumulative protection from distinct H3N2 viruses

OBJECTIVES

Current inactivated influenza vaccines provide suboptimal protection against antigenic drift, and repeated annual vaccinations shape antibody specificity but the effect on protection from infection is not well understood.

METHODS

We studied the effects of cumulative and staggered vaccinations in mice to determine the effect of influenza vaccination on protection from infection and immune quality.

RESULTS

We found that the timing of vaccination and antigenic change impacted the quality of immune responses. When mice received two different H3N2 strains (A/Hong Kong/4801/2014 and A/Singapore/INFIMH-16-0019/2016) by staggered timing of vaccination, there were higher H3HA antibody and B-cell memory responses than four cumulative vaccinations or when two vaccinations were successive. Interestingly, after challenge with a lethal-drifted H3N2 virus (A/Hong Kong/1/1968), mice with staggered vaccination were unable to produce high titres of antibodies specific to the challenge strain compared to other vaccination regimens because of high levels of vaccine-specific cross-reactive antibodies. All vaccination regimens resulted in protection, in terms of viral loads and survival, from lethal challenge, while lung IL-6 and inflammation were lowest in staggered or cumulative vaccination groups, indicating further advantage.

CONCLUSION

Our findings help justify influenza vaccination policies that currently recommend repeat vaccination in infants and annual seasonal vaccination, with no evidence for impaired immunity by repeated seasonal vaccination.

Influenza Vaccine Effectiveness: New Insights and Challenges

Methods for assessing influenza vaccine efficacy and effectiveness have evolved over six decades. Randomized trials remain the gold standard for licensure, but observational studies are needed for annual assessment of vaccine effectiveness (VE). The test-negative design (TND) has become the de facto standard for these field studies. Patients who seek medical care with acute respiratory illness are tested for influenza, and VE is estimated from the odds of vaccination among influenza cases versus test-negative controls. VE varies across seasons, populations, age groups, and products, but VE estimates are consistently higher for A(H1N1)pdm09 and type B compared with A(H3N2). VE studies are increasingly used in combination with molecular epidemiology to understand the viral and immune system factors that drive clinical efficacy and effectiveness. The emerging field of immunoepidemiology offers the potential to understand complex host-virus interactions that affect vaccine protection, and this knowledge will contribute to universal vaccine development.

Impact of influenza vaccination in the Netherlands, 2007-2016: Vaccinees consult their general practitioner for clinically diagnosed influenza, acute respiratory infections, and pneumonia more often than non-vaccinees

Introduction

We aimed to develop an innovative population-based method to estimate the health effect of influenza vaccination based on electronic medical records collected within a general practitioner (GP)-based influenza surveillance system in the Netherlands.

Methods

In each season between 2006/07 and 2015/16, we fitted multilevel Poisson regression models to compare GP consultation rates for clinically diagnosed influenza, acute respiratory infections (ARI), pneumonia, and lower back pain (as a control) between vaccinated vs. unvaccinated individuals. Season-specific relative risks and 95% confidence intervals (CI) were pooled into summary risk ratio (SRR) through random-effects meta-analysis models. Analyses were stratified by patient age (<45, 45–59, 60–74, ≥75 years) and medical indication for the vaccine (any vs. none, subjects aged ≤60 years only).

Results

Overall, 12.6% and 21.4% of study subjects were vaccinated because of their age only or because of an underlying medical condition. Vaccine uptake declined over time, especially among subjects aged ≤74 years with medical indications for vaccination. Vaccinated individuals had significantly higher GP consultation rates for clinically diagnosed influenza (SRR 1.24, 95% CI 1.12–1.38, p-value <0.001), ARI (SRR 1.33, 95% CI 1.27–1.39, p-value <0.001), pneumonia (SRR 1.27, 95% CI 1.19–1.36, p-value <0.001), and lower back pain (SRR 1.21, 95% CI 1.14–1.28, p-value <0.001) compared to unvaccinated individuals.

Discussion

Contrary to expectations, influenza vaccinees have GP consultation rates for clinically diagnosed influenza, ARI and pneumonia that are 24–33% higher compared to unvaccinated individuals. The lower back pain finding suggests that the increase in consultation rates is partially caused by confounding. Importantly, considering the data are not laboratory-confirmed, our results cannot be linked directly to influenza, but only to respiratory illnesses in general.

Impact of prior vaccination on antibody response and influenza-like illness among Australian healthcare workers after influenza vaccination in 2016

BACKGROUND

Epidemiological studies suggest that influenza vaccine effectiveness decreases with repeated administration. We examined antibody responses to influenza vaccination among healthcare workers (HCWs) by prior vaccination history and determined the incidence of influenza infection.

METHODS

HCWs were vaccinated with the 2016 Southern Hemisphere quadrivalent influenza vaccine. Serum samples were collected pre-vaccination, 21-28 days and 7 months post-vaccination. Influenza antibody titres were measured at each time-point using the haemagglutination inhibition (HI) assay. Immunogenicity was compared by prior vaccination history.

RESULTS

A total of 157 HCWs completed the study. The majority were frequently vaccinated, with only 5 reporting no prior vaccinations since 2011. Rises in titres for all vaccine strains among vaccine-naïve HCWs were significantly greater than rises observed for HCWs who received between 1 and 5 prior vaccinations (p < 0.001, respectively). Post-vaccination GMTs against influenza A but not B strains decreased as the number of prior vaccinations increased from 1 to 5. There was a significant decline in GMTs post-season for both B lineages. Sixty five (41%) HCWs reported at least one influenza-like illness episode, with 6 (4%) identified as influenza positive.

CONCLUSIONS

Varying serological responses to influenza vaccination were observed among HCWs by prior vaccination history, with vaccine-naïve HCWs demonstrating greater post-vaccination responses against A(H3N2).

Free ticket, discount ticket or intermediate of the best of two worlds - Which subsidy policy is socially optimal to suppress the disease spreading?

With the aid of the evolutionary vaccination game on a scale-free network, we design a new subsidy policy, named degree dependent subsidy, where cooperative agents get incentives according to their connectivity or degree. That is, agents, having a greater degree, receive a higher incentive, and vice versa. Here we presume that vaccinators are cooperative agents. The new scheme can be said to an intermediate policy between two previously studies policies, namely free ticket and flat discount policies. The former policy distributes free tickets to cooperative hub agents as a priority, whereas the latter dispenses a fixed discount to every cooperator. We compare the efficiency of each policy in terms of having a less infectious state with a minimum social cost. While investigating the performance of the three policies in terms of average social payoff-which takes into account the cost of vaccination as well as infection-the free ticket scheme is found to be the most appealing policies among the three when the budget for subsidy is quite low. The degree dependent subsidy policy outperforms others for a moderate budget size, while the flat discount policy requires a higher budget to effectively suppress the disease. We further estimate threshold levels of the subsidy budget for each policy beyond which subsidizing results in excessive use of vaccination. As a whole, concerning vaccination coverage and final epidemic size, the degree-dependent subsidy scheme outperforms the flat discount scheme, but is dominated by the free ticket policy.

Using Point of Care Testing to estimate influenza vaccine effectiveness in the English primary care sentinel surveillance network

Introduction

Rapid Point of Care Testing (POCT) for influenza could be used to provide information on influenza vaccine effectiveness (IVE) as well as influencing clinical decision-making in primary care.

Methods

We undertook a test negative case control study to estimate the overall and age-specific (6 months-17 years, 18–64 years, ≥65 years old) IVE against medically attended POCT-confirmed influenza. The study took place over the winter of 2019–2020 and was nested within twelve general practices that are part of the Oxford-Royal College of General Practitioners (RCGP) Research and Surveillance Centre (RSC), the English sentinel surveillance network.

Results

648 POCT were conducted. 193 (29.7%) of those who were swabbed had received the seasonal influenza vaccine. The crude unadjusted overall IVE was 46.1% (95% CI: 13.9–66.3). After adjusting for confounders the overall IVE was 26.0% (95% CI: 0–65.5). In total 211 patients were prescribed an antimicrobial after swab testing. Given a positive influenza POCT result, the odds ratio (OR) of receiving an antiviral was 21.1 (95%CI: 2.4–182.2, p = <0.01) and the OR of being prescribed an antibiotic was 0.6 (95%CI: 0.4–0.9, p = <0.01).

Discussion

Using influenza POCT in a primary care sentinel surveillance network to estimate IVE is feasible and provides comparable results to published IVE estimates. A further advantage is that near patient testing of influenza is associated with improvements in appropriate antiviral and antibiotic use. Larger, randomised studies are needed in primary care to see if these trends are still present and to explore their impact on outcomes.

Decline in Seasonal Influenza Vaccine Effectiveness With Vaccination Program Maturation: A Systematic Review and Meta-analysis

BACKGROUND

Evidence suggests that repeated influenza vaccination may reduce vaccine effectiveness (VE). Using influenza vaccination program maturation (PM; number of years since program inception) as a proxy for population-level repeated vaccination, we assessed the impact on pooled adjusted end-season VE estimates from outpatient test-negative design studies.

METHODS

We systematically searched and selected full-text publications from January 2011 to February 2020 (PROSPERO: CRD42017064595). We obtained influenza vaccination program inception year for each country and calculated PM as the difference between the year of deployment and year of program inception. We categorized PM into halves (cut at the median), tertiles, and quartiles and calculated pooled VE using an inverse-variance random-effects model. The primary outcome was pooled VE against all influenza.

RESULTS

We included 72 articles from 11 931 citations. Across the 3 categorizations of PM, a lower pooled VE against all influenza for all patients was observed with PM. Substantially higher reductions were observed in older adults (≥65 years). We observed similar results for A(H1N1)pdm09, A(H3N2), and influenza B.

CONCLUSIONS

The evidence suggests that influenza VE declines with vaccination PM. This study forms the basis for further discussions and examinations of the potential impact of vaccination PM on seasonal VE.