Association of influenza vaccination or influenza virus infection history with subsequent infection risk among children: The Japan Environment and Children's Study (JECS).

We measured the association between history of influenza vaccination by age 2 years and influenza virus (IFV) infection at ages 3 and 4 years by relative risk reduction. We also examined the association between history of IFV infection by age 2 years and recurrent IFV infection at age 3 years. This study included 73,666 children from a large Japanese birth cohort. Among children vaccinated never, once or twice when aged under 2 years, 16.0%, 10.8% and 11.3%, respectively, had been infected with IFV by age 3 years, and 19.2%, 14.5% and 16.0%, respectively, by age 4 years. Compared with no history of influenza vaccination, vaccination at ages 1 and/or 2 years reduced the risk of IFV infection at age 3 by 30%-32% and at age 4 by 17%-24%. The relative risk of recurrent IFV infection at ages 3 and 4 years increased in proportion to the number of prior infections by age 2. One-season-prior influenza vaccination history reduced the IFV infection risk at age 3 years by 25%-42%. Influenza vaccination most effectively protected children at age 3 who lacked older sibling(s) and did not attend nursery school. One-season-prior IFV infection increased the relative risk of recurrent infection at age 3 years (1.72-3.33). In conclusion, influenza vaccination-induced protection may partly extend to the next season. Owing to the relative risk reduction by influenza vaccination and the increased relative risk of IFV infection from prior-season infection, annual influenza vaccination is recommended.

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.

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

BackgroundMost reports of influenza vaccine effectiveness consider current-season vaccination only.AimWe evaluated a method to estimate the effect of influenza vaccinations (EIV) considering vaccination history.MethodsWe 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.ResultsThe 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.ConclusionsVaccine 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.

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).

A Population-Based Propensity Score-Matched Study to Assess the Impact of Repeated Vaccination on Vaccine Effectiveness for Influenza-Associated Hospitalization Among the Elderly.

BACKGROUND: Influenza is a major cause of morbidity and mortality in the elderly worldwide. Influenza vaccination can prevent morbidity/mortality from influenza infection. A gap of 1-2 years, before an epidemic strain is recommended by the World Health Organization (WHO) to be the vaccine strain in Southeast Asia, has been reported; this results in a high rate of vaccine mismatch and excess influenza-associated morbidity. The aim of the current study was to evaluate the effect of repeated vaccination on vaccine effectiveness (VE) among the elderly in Taiwan, during years with and without early appearance of antigenically drifted strains. METHODS: A historical cohort study was conducted to evaluate the impact of repeated vaccination on the reduction of influenza-associated hospitalization among persons older than 64 years over two influenza seasons: 2007-08, with all circulating virus strains mismatched, and 2008-09, with all virus strains matched with the vaccine strains, considering four exposure effects, namely current vaccine effect, sequential vaccination effect, residual protection effect and no vaccination effect. Propensity score matching on vaccination status was performed to ensure similar baseline characteristics between the groups that received and did not receive vaccination. RESULTS: Only current-year vaccination in combination with prior history of annual revaccination significantly reduced the risk of hospitalization, with adjusted hazard ratios of 0.68 (95% CI: 0.54, 0.85) and 0.74 (95% CI: 0.57, 0.95) during the 2007-08 and 2008-09 influenza seasons, respectively. Further stratification showed that even during the 2007-08 influenza season, when all vaccinations were mismatched with the circulating strains, sequential vaccinations still significantly reduced influenza-associated hospitalization in the female population aged 68-74 and 75-84 years, with adjusted VE of 25.2% (95% CI: -9.6, 49.0%) and 36.9% (95% CI: 17.1, 52.0%), respectively. CONCLUSION: Our study supports the recommendation of annual revaccination against influenza in the elderly, even though the circulating strain of influenza virus was antigenically mismatched with the vaccine strains.

Effects of prior influenza virus vaccination on maternal antibody responses: Implications for achieving protection in the newborns.

BACKGROUND: In the US, influenza vaccination is recommended annually to everyone ≥6months. Prior receipt of influenza vaccine can dampen antibody responses to subsequent vaccination. This may have implications for pregnant women and their newborns, groups at high risk for complications from influenza infection. OBJECTIVE: This study examined effects of prior vaccination on maternal and cord blood antibody levels in a cohort of pregnant women in the US. STUDY DESIGN: Influenza antibody titers were measured in 141 pregnant women via the hemagglutination inhibition (HAI) assay prior to receipt of quadrivalent influenza vaccine, 30days post-vaccination, and at delivery (maternal and cord blood). Logistic regression analyses adjusting for age, BMI, parity, gestational age at vaccination, and year of vaccination compared HAI titers, seroprotection, and seroconversion in women with versus without vaccination in the prior year. RESULTS: Compared to those without vaccination in the previous year (n=50), women with prior vaccination (n=91) exhibited higher baseline antibody titers and/or seroprotection rates against all four strains after controlling for covariates. Prior vaccination also predicted lower antibody responses and seroconversion rates at one month post-vaccination. However, at delivery, there were no significant differences in antibody titers or seroprotection rates in women or newborns, and no meaningful differences in the efficiency of antibody transfer, as indicated by the ratio of cord blood to maternal antibody titers at the time of delivery. CONCLUSION: In this cohort of pregnant women, receipt of influenza vaccine the previous year predicted higher baseline antibody titers and decreased antibody responses at one month post-vaccination against all influenza strains. However, prior maternal vaccination did not significantly affect either maternal antibody levels at delivery or antibody levels transferred to the neonate. This study is registered with the NIH as a clinical trial (NCT02148874).

Concurrent and cross-season protection of inactivated influenza vaccine against A(H1N1)pdm09 illness among young children: 2012-2013 case-control evaluation of influenza vaccine effectiveness.

In 2012-2013, we examined 1729 laboratory-confirmed A(H1N1)pdm09 influenza cases matched 1:1 with healthy controls and estimated influenza vaccine effectiveness (VE) for trivalent inactivated influenza vaccine (IIV3) to be 67% (95% confidence interval=58-74%) for ages 8 months to 6 years old. Among children aged 8-35 months old, VE for fully vaccinated children (73%, 60-81%) was significantly higher than VE for partially vaccinated children (55%, 33-70%). Significant cross-season protection from prior IIV3 was noted, including VE of 31% (8-48%) from IIV3 received in 2010-2011 against influenza illness in 2012--2013 without subsequent boosting doses.