Effectiveness of influenza vaccine against influenza A in Europe in seasons of different A(H1N1)pdm09 and the same A(H3N2) vaccine components (2016-17 and 2017-18)

Influenza A(H3N2) viruses predominated in Europe in 2016-17. In 2017-18 A(H3N2) and A(H1N1)pdm09 viruses co-circulated. The A(H3N2) vaccine component was the same in both seasons; while the A(H1N1)pdm09 component changed in 2017-18. In both seasons, vaccine seed A(H3N2) viruses developed adaptations/alterations during propagation in eggs, impacting antigenicity. We used the test-negative design in a multicentre primary care case-control study in 12 European countries to measure 2016-17 and 2017-18 influenza vaccine effectiveness (VE) against laboratory-confirmed influenza A(H1N1)pdm09 and A(H3N2) overall and by age group. During the 2017-18 season, the overall VE against influenza A(H1N1)pdm09 was 59% (95% CI: 47-69). Among those aged 0-14, 15-64 and ≥65 years, VE against A(H1N1)pdm09 was 64% (95% CI: 37-79), 50% (95% CI: 28-66) and 66% (95% CI: 42-80), respectively. Overall VE against influenza A(H3N2) was 28% (95% CI: 17-38) in 2016-17 and 13% (95% CI: -15 to 34) in 2017-18. Among 0-14-year-olds VE against A(H3N2) was 28% (95%CI: -10 to 53) and 29% (95% CI: -87 to 73), among 15-64-year-olds 34% (95% CI: 18-46) and 33% (95% CI: -3 to 56) and among those aged ≥65 years 15% (95% CI: -10 to 34) and -9% (95% CI: -74 to 32) in 2016-17 and 2017-18, respectively. Our study suggests the new A(H1N1)pdm09 vaccine component conferred good protection against circulating strains, while VE against A(H3N2) was