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00925njm 22002777a 4500 dc Kissling, Esther author Pozo Sanchez, Francisco author Buda, Silke author Vilcu, Ana-Maria author Gherasim, Alin Manuel author Brytting, Mia author Domegan, Lisa author Gomez, Verónica author Meijer, Adam author Lazar, Mihaela author Vučina, Vesna Višekruna author Dürrwald, Ralf author van der Werf, Sylvie author Larrauri, Amparo author Enkirch, Theresa author O'Donnell, Joan author Guiomar, Raquel author Hooiveld, Mariëtte author Petrović, Goranka author Stoian, Elena author Penttinen, Pasi author Valenciano, Marta author I-Move Primary Care Study Team author Mazagatos, Clara author Casas Flecha, Inmaculada author 2019-11 Introduction: 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 A(H3N2) VE by age and genetic subgroups to explore VE by birth cohort. Methods: We 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. Results: Influenza 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. Discussion: The 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. Euro Surveill. 2019 Nov;24(48):1900604. 10.2807/1560-7917.ES.2019.24.48.1900604 1560-7917 1560-7917 Euro surveillance : bulletin Europeen sur les maladies transmissibles = European communicable disease bulletin 31796152 http://hdl.handle.net/20.500.12105/8975 A(H3N2) Birth cohorts Imprinting Influenza Multicentre study Vaccine effectiveness Low 2018/19 vaccine effectiveness against influenza A(H3N2) among 15-64-year-olds in Europe: exploration by birth cohort