Abstract
IntroductionInfluenza A(H3N2) clades 3C.2a and 3C.3a co-circulated in Europe in 2018/19. Immunological imprinting by first childhood influenza infection may induce future birth cohort differences in vaccine effectiveness (VE).AimThe I-MOVE multicentre primary care test-negative study assessed 2018/19 influenza A(H3N2) VE by age and genetic subgroups to explore VE by birth cohort.MethodsWe measured VE against influenza A(H3N2) and (sub)clades. We stratified VE by usual age groups (0–14, 15–64, ≥ 65-years). To assess the imprint-regulated effect of vaccine (I-REV) hypothesis, we further stratified the middle-aged group, notably including 32–54-year-olds (1964–86) sharing potential childhood imprinting to serine at haemagglutinin position 159.ResultsInfluenza A(H3N2) VE among all ages was −1% (95% confidence interval (CI): −24 to 18) and 46% (95% CI: 8–68), −26% (95% CI: −66 to 4) and 20% (95% CI: −20 to 46) among 0–14, 15–64 and ≥ 65-year-olds, respectively. Among 15–64-year-olds, VE against clades 3C.2a1b and 3C.3a was 15% (95% CI: −34 to 50) and −74% (95% CI: −259 to 16), respectively. VE was −18% (95% CI: −140 to 41), −53% (95% CI: −131 to −2) and −12% (95% CI: −74 to 28) among 15–31-year-olds (1987–2003), 32–54-year-olds (1964–86) and 55–64-year-olds (1954–63), respectively.DiscussionThe lowest 2018/19 influenza A(H3N2) VE was against clade 3C.3a and among those born 1964–86, corresponding to the I-REV hypothesis. The low influenza A(H3N2) VE in 15–64-year-olds and the public health impact of the I-REV hypothesis warrant further study.
Highlights
Influenza A(H3N2) clades 3C.2a and 3C.3a co-circulated in Europe in 2018/19
Using the I-MOVE multicentre case control study (MCCS), we report 2018/19 end-ofseason vaccine effectiveness (VE) against influenza A(H3N2), overall, by age group and for A(H3N2)clades
Results suggest that this was driven by the age group 15–64 years, in whom the
Summary
Influenza A(H3N2) clades 3C.2a and 3C.3a co-circulated in Europe in 2018/19. Immunological imprinting by first childhood influenza infection may induce future birth cohort differences in vaccine effectiveness (VE). Aim: The I-MOVE multicentre primary care test-negative study assessed 2018/19 influenza. We stratified VE by usual age groups (0–14, 15–64, ≥ 65-years). To assess the imprint-regulated effect of vaccine (I-REV) hypothesis, we further stratified the middle-aged group, notably including 32–54-year-olds (1964–86) sharing potential childhood imprinting to serine at haemagglutinin position. The 2018/19 influenza season in Europe was characterised by both A(H1N1)pdm and A(H3N2) virus subtypes circulating, with co-circulation in some countries and dominance of either A(H1N1)pdm or A(H3N2). Number of ILI patients by influenza A(H3N2) status (test-negative controls and A(H3N2) cases) and week of symptom onset, I-MOVE primary care multicentre study, Europe, influenza season 2018/19 (n = 6,172).
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