Early estimates of seasonal influenza vaccine effectiveness in Europe: results from the I-MOVE multicentre case-control study, 2012/13

Baltazar Nunes: member of the I-MOVE case–control study teamWe conducted a test-negative case–control study based in five European sentinel surveillance networks. The early 2012/13 adjusted influenza vaccine effectiveness was 78.2% (95% CI: 18.0 to 94.2) against influenza B, 62.1% (95% CI: −22.9 to 88.3%) against A(H1)pdm09, 41.9 (95% CI: −67.1 to 79.8) against A(H3N2) and 50.4% (95% CI: −20.7 to 79.6) against all influenza types in the target groups for vaccination. Efforts to improve influenza vaccines should continue to better protect those at risk of severe illness or complications

I-MOVE multicentre case–control study 2010/11 to 2014/15 : is there within-season waning of influenza type/subtype vaccine effectiveness with increasing time since vaccination?

Influenza vaccines are currently the best method available to prevent seasonal influenza infection. In most European countries one dose (or two doses for children) of seasonal vaccine is given from September to December to the elderly and other target groups for vaccination. In Europe, influenza seasons can last until mid-May (1), and it is expected that vaccination conveys protection on the individual for the duration of the season. In 13/15 reviewed studies on the length of vaccine-induced protection among the elderly, using anti-haemagglutination antibody titres as a proxy for seroprotection levels, seroprotection rates lasted at least >4 months after vaccination (2). However in the 2011-12 influenza season various studies in Europe reported a decrease in influenza vaccine effectiveness (VE) against A(H3N2) over time within the season (3–5). In the United States, a decrease in VE against A(H3N2) with time since vaccination was suggested in the 2007-8 influenza season (6). The observed decrease of VE over time can be explained by viral change (notably antigenic drift) occurring in the season. Drift in B viruses may be slower than in A viruses (7), and A(H3N2) viruses undergo antigenic drift more frequently than A(H1N1)pdm09 viruses (8). The decrease of VE over time can also be explained by a waning of the immunity conferred by the vaccine independently from viral changes. If vaccine-induced protection wanes more rapidly during the season, then depending on the start and duration of the influenza season, the decline of VE may cause increases in overall incidence, hospitalisations and deaths. Changes to vaccination strategies (timing and boosters) may be needed. As anti-haemagglutination antibody titres are not well defined as a correlate of protection (9,10), vaccine efficacy (as measured in trials) or vaccine effectiveness observational studies may be one way to measure vaccine-induced protection. These studies require a large sample size to model VE by time since vaccination and currently, most of the seasonal observational studies lack the precision required to provide evidence for waning immunity. In this study we pooled data across five post-pandemic seasons (2010/11-2014/15) from the I-MOVE (Influenza - Monitoring Vaccine Effectiveness) multicentre case control studies (1,3,11,12), to obtain a greater sample size to study the effects of time since vaccination on influenza type/subtype-specific VE. We measure influenza type/subtype-specific VE by time since vaccination for the overall season, but also in the early influenza phase; under the hypothesis that virological changes are fewer in the early season, but waning of the vaccine effect should be present regardless of time within the influenza phase

Early estimates of seasonal influenza vaccine effectiveness in Europe: results from the I-MOVE multicentre case–control study, 2012/13

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The European I-MOVE Multicentre 2013–2014 Case-Control Study. Homogeneous moderate influenza vaccine effectiveness against A(H1N1)pdm09 and heterogenous results by country against A(H3N2)

AbstractBackgroundIn the first five I-MOVE (Influenza Monitoring Vaccine Effectiveness in Europe) influenza seasons vaccine effectiveness (VE) results were relatively homogenous among participating study sites. In 2013–2014, we undertook a multicentre case-control study based on sentinel practitioner surveillance networks in six European Union (EU) countries to measure 2013–2014 influenza VE against medically-attended influenza-like illness (ILI) laboratory-confirmed as influenza. Influenza A(H3N2) and A(H1N1)pdm09 viruses co-circulated during the season.MethodsPractitioners systematically selected ILI patients to swab within eight days of symptom onset.We compared cases (ILI positive to influenza A(H3N2) or A(H1N1)pdm09) to influenza negative patients. We calculated VE for the two influenza A subtypes and adjusted for potential confounders. We calculated heterogeneity between sites using the I2 index and Cochrane's Q test. If the I2 was <50%, we estimated pooled VE as (1 minus the OR)×100 using a one-stage model with study site as a fixed effect. If the I2 was >49% we used a two-stage random effects model.ResultsWe included in the A(H1N1)pdm09 analysis 531 cases and 1712 controls and in the A(H3N2) analysis 623 cases and 1920 controls. For A(H1N1)pdm09, the Q test (p=0.695) and the I2 index (0%) suggested no heterogeneity of adjusted VE between study sites. Using a one-stage model, the overall pooled adjusted VE against influenza A(H1N1)pdm2009 was 47.5% (95% CI: 16.4–67.0).For A(H3N2), the I2 was 51.5% (p=0.067). Using a two-stage model for the pooled analysis, the adjusted VE against A(H3N2) was 29.7 (95% CI: −34.4–63.2).ConclusionsThe results suggest a moderate 2013–2014 influenza VE against A(H1N1)pdm09 and a low VE against A(H3N2). The A(H3N2) estimates were heterogeneous among study sites. Larger sample sizes by study site are needed to prevent statistical heterogeneity, decrease variability and allow for two-stage pooled VE for all subgroup analyses