Influenza vaccine effectiveness against hospital-attended influenza infection in 2023/24 season in Hangzhou, China

BackgroundFrom 2020, influenza activities were largely affected by the coronavirus disease (COVID-19) pandemic at the global scale. The B/Yamagata lineage has become extinct since 2020, raising concerns regarding the quadrivalent influenza vaccine. Evaluating vaccine effectiveness (VE) against influenza infections is important to guide future influenza vaccine programs. MethodsA test-negative case-control study was conducted in five tertiary hospitals in Hangzhou, the capital city of Zhejiang province, China. Hospital-attended patients aged >6 months who presented with influenza-like illness (ILI) from October 1, 2023 to March 31, 2024 were enrolled in this study. The VE was estimated using multivariate logistic regression models, adjusted by sex, age, influenza detection methods and influenza testing timing. ResultsIn total, 157291 hospital-attended ILI participants were enrolled. 56704 (36%) were tested positive for influenza virus. The adjusted estimates of overall VE against any hospital-attended influenza infection was 48% (95% Confidence interval [CI]: 46%-51%). The overall VE of trivalent inactivated influenza vaccine (IIV3) was 59% (95% CI: 50%-66%), followed by trivalent live attenuated vaccine (LAIV3) (VE=53%, 95% CI: 42%-62%) and quadrivalent inactivated influenza vaccine (IIV4) (VE=47%, 95% CI: 45%-50%). IIV3 provided even much better protection against hospital-attended influenza B infection than IIV4 (VE=87% (95% CI: 81%-92%) for IIV3 versus VE=53%, 95% CI: 50%-57% for IIV4). ConclusionsThe influenza vaccine provided moderate protection against influenza infection in the 2023/24 season in Hangzhou, China, during a massive epidemic. The results supported the World Health Organization recommendation regarding the exclusion of B/Yamagata lineage antigen in quadrivalent influenza vaccines in 2023.