Impact of repeated influenza vaccinations in persons over 65 years of age: A large population-based cohort study of severe influenza over six consecutive seasons, 2011/12-2016/17

A Multiseason Randomized Controlled Trial of Advax-Adjuvanted Seasonal Influenza Vaccine in Participants With Chronic Disease or Older Age

BACKGROUND

The aim of the current study was to determine the safety and immunogenicity of trivalent inactivated influenza vaccine (TIV) alone or formulated with Advax delta inulin adjuvant in those who were older (aged >60 years) or had chronic disease.

METHODS

Over 4 consecutive years from 2008 through 2011, adult participants with chronic disease or >60 years of age were recruited into a randomized controlled study to assess the safety, tolerability and immunogenicity of Advax-adjuvanted TIV (TIV + Adj) versus standard TIV. The per-protocol population with ≥1 postbaseline measurement of influenza antibodies comprised 1297 participants, 447 in the TIV and 850 in the TIV + Adj) group.

RESULTS

No safety issues were identified. Variables negatively affecting vaccine responses included obesity and diabetes mellitus. Advax adjuvant had a positive impact on anti-influenza immunoglobulin M responses and on H3N2 and B strain seropositivity as assessed by hemagglutination inhibition.

CONCLUSIONS

TIV + Adj was safe and well tolerated in individuals with chronic disease. There is an ongoing need for research into improved influenza vaccines for high-risk populations.

CLINICAL TRIALS REGISTRATION

Australia New Zealand Clinical Trial Registry: ACTRN 12608000364370.

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.

Relationship between Humoral Response in COVID-19 and Seasonal Influenza Vaccination

There is evidence that vaccination against seasonal influenza can improve innate immune responses to COVID-19 and decrease disease severity. However, less is known about whether it could also impact the humoral immunity in SARS-CoV-2 infected patients. The present study aimed to compare the SARS-CoV-2 specific humoral responses (IgG antibodies against nucleocapsid; anti-N, receptor binding domain; anti-RBD, subunit S2; anti-S2, and envelope protein; anti-E) between non-hospitalized, COVID-19 unvaccinated, and mild COVID-19 convalescent patients who were and were not vaccinated against influenza during the 2019/2020 epidemic season (n = 489 and n = 292, respectively). The influenza-vaccinated group had significantly higher frequency and titers of anti-N antibodies (75 vs. 66%; mean 559 vs. 520 U/mL) and anti-RBD antibodies (85 vs. 76%; mean 580 vs. 540 U/mL). The prevalence and concentrations of anti-S2 and anti-E antibodies did not differ between groups (40-43%; mean 370-375 U/mL and 1.4-1.7%; mean 261-294 U/mL) and were significantly lower compared to those of anti-RBD and anti-N. In both groups, age, comorbidities, and gender did not affect the prevalence and concentrations of studied antibodies. The results indicate that influenza vaccination can improve serum antibody levels produced in response to SARS-CoV-2 infection.

Effectiveness and cost-effectiveness of influenza vaccination for elderly people

For elderly people who have low incidence of influenza, calculation of credible vaccine effectiveness (VE) sometimes becomes difficult. Currently, VE for elderly people is insufficient to ascertain the precise efficacy specifically. Cost-effectiveness of influenza vaccination of elderly people is discussed widely in terms of topics and areas. This report describes research results demonstrating influenza vaccination effectiveness among elderly people based on recent findings. Newly available influenza vaccination for elderly people appears to be cost-effective compared with that of trivalent inactiveted influenza vaccine. Overall, for all influenza virus types, it remains unclear whether influenza vaccination shows high VE. A decreasing effect of repeated vaccination was confirmed partially by test negative design and a serological study of cohorts. However, some studies have found no such decreasing effect. Measurement of VE and subsequent analysis of the cost-effectiveness of influenza vaccination for elderly people requires long-term monitoring using serological studies and test negative design.

Sex-specific effects of aging on humoral immune responses to repeated influenza vaccination in older adults

Older adults (≥65 years of age) bear a significant burden of severe disease and mortality associated with influenza, despite relatively high annual vaccination coverage and substantial pre-existing immunity to influenza. To test the hypothesis that host factors, including age and sex, play a role in determining the effect of repeated vaccination and levels of pre-existing humoral immunity to influenza, we evaluated pre- and post-vaccination strain-specific hemagglutination inhibition (HAI) titers in adults over 75 years of age who received a high-dose influenza vaccine in at least four out of six influenza seasons. Pre-vaccination titers, rather than host factors and repeated vaccination were significantly associated with post-vaccination HAI titer outcomes, and displayed an age-by-sex interaction. Pre-vaccination titers to H1N1 remained constant with age. Titers to H3N2 and influenza B viruses decreased substantially with age in males, whereas titers in females remained constant with age. Our findings highlight the importance of pre-existing immunity in this highly vaccinated older adult population and suggest that older males are particularly vulnerable to reduced pre-existing humoral immunity to influenza.

Spotlight influenza: Estimation of influenza vaccine effectiveness in elderly people with assessment of residual confounding by negative control outcomes, Finland, 2012/13 to 2019/20

Background

Cohort studies on vaccine effectiveness are prone to confounding bias if the distribution of risk factors is unbalanced between vaccinated and unvaccinated study subjects.

Aim

We aimed to estimate influenza vaccine effectiveness in the elderly population in Finland by controlling for a sufficient set of confounders based on routinely available register data.

Methods

For each of the eight consecutive influenza seasons from 2012/13 through 2019/20, we conducted a cohort study comparing the hazards of laboratory-confirmed influenza in vaccinated and unvaccinated people aged 65–100 years using individual-level medical and demographic data. Vaccine effectiveness was estimated as 1 minus the hazard ratio adjusted for the confounders age, sex, vaccination history, nights hospitalised in the past and presence of underlying chronic conditions. To assess the adequacy of the selected set of confounders, we estimated hazard ratios of off-season hospitalisation for acute respiratory infection as a negative control outcome.

Results

Each analysed cohort comprised around 1 million subjects, of whom 37% to 49% were vaccinated. Vaccine effectiveness against laboratory-confirmed influenza ranged from 16% (95% confidence interval (CI): 12–19) to 48% (95% CI: 41–54). More than 80% of the laboratory-confirmed cases were hospitalised. The adjusted off-season hazard ratio estimates varied between 1.00 (95% CI: 0.94–1.05) and 1.08 (95% CI: 1.01–1.15), indicating that residual confounding was absent or negligible.

Conclusion

Seasonal influenza vaccination reduces the hazard of severe influenza disease in vaccinated elderly people. Data about age, sex, vaccination history and utilisation of hospital care proved sufficient to control confounding.

Impact of influenza vaccination on amoxicillin prescriptions in older adults: A retrospective cohort study using primary care data

Background

Bacterial infections of the upper and lower respiratory tract are a frequent complication of influenza and contribute to the widespread use of antibiotics. Influenza vaccination may help reduce both appropriate and inappropriate prescribing of antibiotics. Electronic health records provide a rich source of information for assessing secondary effects of influenza vaccination.

Methods

We conducted a retrospective study to estimate effects of influenza vaccine on antibiotic (amoxicillin) prescription in the elderly based on data from the Clinical Practice Research Datalink. The introduction of UK policy to recommend the influenza vaccine to older adults in 2000 led to a substantial increase in uptake, creating a natural experiment. Of 259,753 eligible patients that were unvaccinated in 1999 and aged≥65y by January 2000, 88,519 patients received influenza vaccination in 2000. These were propensity score matched 1:1 to unvaccinated patients. Time-to-amoxicillin was analysed using the Prior Event Rate Ratio (PERR) Pairwise method to address bias from time-invariant measured and unmeasured confounders. A simulation study and negative control outcome were used to help strengthen the validity of results.

Results

Compared to unvaccinated patients, those from the vaccinated group were more likely to be prescribed amoxicillin in the year prior to vaccination: hazard ratio (HR) 1.90 (95% confidence interval 1.83, 1.98). Following vaccination, the vaccinated group were again more likely to be prescribed amoxicillin, HR 1.64 (1.58,1.71). After adjusting for prior differences between the two groups using PERR Pairwise, overall vaccine effectiveness was 0.86 (0.81, 0.92). Additional analyses suggested that provided data meet the PERR assumptions, these estimates were robust.

Conclusions

Once differences between groups were taken into account, influenza vaccine had a beneficial effect, lowering the frequency of amoxicillin prescribing in the vaccinated group. Ensuring successful implementation of national programmes of vaccinating older adults against influenza may help contribute to reducing antibiotic resistance.

Influenza Vaccine Effectiveness and Waning Effect in Hospitalized Older Adults. Valencia Region, Spain, 2018/2019 Season

Influenza vaccination is annually recommended for specific populations at risk, such as older adults. We estimated the 2018/2019 influenza vaccine effectiveness (IVE) overall, by influenza subtype, type of vaccine, and by time elapsed since vaccination among subjects 65 years old or over in a multicenter prospective study in the Valencia Hospital Surveillance Network for the Study of Influenza and other Respiratory Viruses (VAHNSI, Spain). Information about potential confounders was obtained from clinical registries and/or by interviewing patients and vaccination details were only ascertained by registries. A test-negative design was performed in order to estimate IVE. As a result, IVE was estimated at 46% (95% confidence interval (CI): (16%, 66%)), 41% (95% CI: (−34%, 74%)), and 45% (95% CI: (7%, 67%)) against overall influenza, A(H1N1)pdm09 and A(H3N2), respectively. An intra-seasonal not relevant waning effect was detected. The IVE for the adjuvanted vaccine in ≥75 years old was 45% (2%, 69%) and for the non-adjuvanted vaccine in 65–74 years old was 59% (−16%, 86%). Thus, our data revealed moderate vaccine effectiveness against influenza A(H3N2) and not significant against A(H1N1)pdm09. Significant protection was conferred by the adjuvanted vaccine to patients ≥75 years old. Moreover, an intra-seasonal not relevant waning effect was detected, and a not significant IVE decreasing trend was observed over time.

Impact of repeated yearly vaccination on immune responses to influenza vaccine in an elderly population

BACKGROUND

The effectiveness of repeated vaccination for seasonal influenza remains controversial. Here, we measured antibody responses to the influenza virus (A/H1N1, A/H3N2 and B) in a closed cohort of older participants vaccinated against influenza virus in each of 5 consecutive years.

METHODS

One hundred and 11 volunteers aged >61 years were vaccinated subcutaneously with 1 dose (0.5 ml) of inactivated influenza vaccine as recommended by the World Health Organization from the 2005-2006 season through the 2009-2010 season. Hemagglutination inhibition (HI) antibody titers were determined.

RESULTS

HI antibody titers against all 3 virus strains were significantly higher at 4 weeks after vaccination than at a time point prior to vaccination in each of the 5 seasons (P < .01); HI antibody titers were detected at the original prevaccination levels just prior to re-vaccination the following year. Sero-protection and HI antibody titers at 4 weeks after vaccination were similar against all influenza strains and during most of the 5 seasons evaluated. Vaccine strain changes were associated with specific immune responses in 9 of 12 (75%) intervals.

CONCLUSIONS

Taken together, our results suggest that annual vaccination is necessary to maintain humoral immunity for the elderly population. Furthermore, our findings revealed that annual seasonal vaccination was not associated with reduced vaccine effectiveness, and that the reformation of the vaccine resulted in amplified immune responses among those undergoing yearly vaccination in the elderly population.

The impact of repeated vaccination using 10-year vaccination history on protection against influenza in older adults: a test-negative design study across the 2010/11 to 2015/16 influenza seasons in Ontario, Canada

Introduction

Annual influenza vaccination is recommended for older adults, but evidence regarding the impact of repeated vaccination has been inconclusive.

Aim

We investigated vaccine effectiveness (VE) against laboratory-confirmed influenza and the impact of repeated vaccination over 10 previous seasons on current season VE among older adults.

Methods

We conducted an observational test-negative study in community-dwelling adults aged > 65 years in Ontario, Canada for the 2010/11 to 2015/16 seasons by linking laboratory and health administrative data. We estimated VE using multivariable logistic regression. We assessed the impact of repeated vaccination by stratifying by previous vaccination history.

Results

We included 58,304 testing episodes for respiratory viruses, with 11,496 (20%) testing positive for influenza and 31,004 (53%) vaccinated. Adjusted VE against laboratory-confirmed influenza for the six seasons combined was 21% (95% confidence interval (CI): 18 to 24%). Patients who were vaccinated in the current season, but had received no vaccinations in the previous 10 seasons, had higher current season VE (34%; 95%CI: 9 to 52%) than patients who had received 1–3 (26%; 95%CI: 13 to 37%), 4–6 (24%; 95%CI: 15 to 33%), 7–8 (13%; 95%CI: 2 to 22%), or 9–10 (7%; 95%CI: −4 to 16%) vaccinations (trend test p = 0.001). All estimates were higher after correcting for misclassification of current season vaccination status. For patients who were not vaccinated in the current season, residual protection rose significantly with increasing numbers of vaccinations received previously.

Conclusions

Although VE appeared to decrease with increasing numbers of previous vaccinations, current season vaccination likely provides some protection against influenza regardless of the number of vaccinations received over the previous 10 influenza seasons.

Semiannual Versus Annual Influenza Vaccination in Older Adults in the Tropics: An Observer-blind, Active-comparator-controlled, Randomized Superiority Trial

BACKGROUND

Antibody titres and vaccine effectiveness decline within 6 months after influenza vaccination in older adults. Biannual vaccination may be necessary to provide year-round protection in the tropics, where influenza circulates throughout the year.

METHODS

Tropical Influenza Control Strategies (TROPICS1) was a single-center, 1:1 randomized, observer-blinded, active-comparator-controlled, superiority study in 200 community-resident adults aged ≥65 years. Participants received a standard-dose trivalent inactivated influenza vaccination (IIV3) at enrollment, and either tetanus-diphtheria-pertussis vaccination or IIV3 6 months later. The primary outcome was the proportion of participants with haemagglutination-inhibition (HI) geometric mean titre (GMT) ≥1:40 1 month after the second vaccination (month 7). Secondary outcomes included GMTs to month 12, the incidence of influenza-like illness (ILI), and adverse reactions after vaccination.

RESULTS

At month 7, the proportion of participants with an HI tire ≥1:40 against A/H1N1 increased by 21.4% (95% confidence interval [CI] 8.6-33.4) in the semiannual vaccination group. This proportion was not significantly higher for A/H3N2 (4.3, 95% CI -1.1-10.8) or B (2.1, 95% CI -2.0-7.3). Semiannual vaccination significantly increased GMTs against A/H1N1 and A/H3N2, but not B, at month 7. Participants receiving a repeat vaccination of IIV3 reported a significantly lower incidence of ILI in the 6 months after the second vaccination (relative vaccine effectiveness 57.1%, 95% CI 0.6-81.5). The frequency of adverse events was similar after the first and second influenza vaccinations.

CONCLUSIONS

Semiannual influenza vaccination in older residents of tropical countries has the potential to improve serological measures of protection against infection. Alternative vaccination strategies should also be studied.

CLINICAL TRIALS REGISTRATION

NCT02655874.

Effectiveness of repeated influenza vaccination among the elderly population with high annual vaccine uptake rates during the three consecutive A/H3N2 epidemics

BACKGROUND

Annually, about 80% of the Korean elderly aged ≥65 years receive influenza vaccination. Repeated annual vaccination has been suggested as an important factor of poor influenza vaccine effectiveness (VE), though reported conflicting results.

METHODS

During the consecutive A/H3N2-dominant influenza seasons between 2012 and 2015, we comparatively evaluated the VE (repeated vs. current season only) against laboratory-confirmed influenza, pneumonia and hospitalization in the elderly aged ≥65 years with influenza-like illness (ILI). Clinical and demographic data were collected prospectively, and vaccination status of prior and current seasons was verified using the immunization registry data of Korean Centers for Disease Control and Prevention.

RESULTS

During the first A/H3N2-dominant season in 2012-2013, influenza vaccine showed statistically significant effectiveness against influenza A infection only and when vaccinated in the current season only (VE 53%, 95% CI 15-77). In the latter two seasons (2013-2015 years), the adjusted VE for influenza A was indistinguishable between repeated vaccination and vaccination in the current season only.

CONCLUSION

During consecutive influenza A/H3N2 epidemics, poor influenza vaccine effectiveness may be more pronounced among the elderly population with a high annual vaccine uptake rate.

Determinants of vaccination uptake, and influenza vaccine effectiveness in preventing deaths and hospital admissions in the elderly population; Treviso, Italy, 2014/2015-2016/2017 seasons

Seasonal influenza is an important cause of morbidity and mortality, particularly among the elderly population. Determinants of vaccination uptake and its impact on health outcomes in the seasons 2014/2015–2016/2017 in elderly living in Treviso area (Veneto Region, North-Eastern Italy) were evaluated. A retrospective cohort study was conducted combining information from several health administrative databases, and multiple Poisson regression models were applied to evaluate the influenza vaccine effectiveness, also adjusting for confounding factors. MF59-adjuvanted trivalent-inactivated vaccine was mainly administered. Data from more than 83,000 elderly people were analyzed by year. Vaccine coverage was about 50%; influenza vaccination uptake was independently associated with older age, male sex, increasing number of underlying chronic conditions, previous pneumococcal vaccination, annual expenses for specialist medical cares, and general practitioner to whom the elderly was in charge. After adjusting for previously described characteristics, vaccination was associated with lower mortality and influenza-related hospitalization rates. Specifically, during influenza season the adjusted incidence rate ratio of death and of influenza-related hospitalizations for vaccinated compared to unvaccinated persons was 0.63 [95% confidence interval (CI) 0.58–0.69, p < .001] and 0.86 (95% CI 0.81–0.91, p < .001), respectively. A similar effectiveness was estimated for death in all age groups (≤74, 75–84, ≥85 years old), whereas a higher effect was found for hospitalizations in subjects aged ≥75 years old. Vaccination was also effective both in males and females. Findings suggest a health benefit of the influenza vaccination in the elderly population. Efforts should be focused on strategies to increase the vaccination uptake as important instrument of prevention.