Influenza Vaccination Effectiveness Against Influenza-Associated Hospitalization in Children and the Effects of...

Given concerns that influenza vaccine effectiveness (VE) might differ with repeated annual vaccination, we estimated VE against influenza-associated hospitalization and assessed repeat vaccination effects.

We analyzed a test-negative design study conducted in 3 Hong Kong hospitals (October 2015 - July 2025), excluding the 2020/21 and 2021/22 seasons due to the absence of influenza circulation during the COVID-19 pandemic. Polymerase chain reaction testing was used to identify influenza virus infections. We used conditional logistic regression to estimate influenza VE overall, by influenza type/subtype, and by influenza vaccination status in the preceding year (repeat vaccination status).

We analyzed data on 34 237 children, among whom 5245 (15.3%) tested positive for influenza. VE against influenza-associated hospitalization was 57.2% (95% confidence interval [CI]: 52.3%, 61.6%), with subtype-specific VE estimates of 67.7% (95% CI: 61.8%, 72.7%) for A(H1N1)pdm09, 60.6% (95% CI: 50.8%, 68.5%) for influenza B, and 37.2% (95% CI: 24.7%, 47.6%) for A(H3N2). The overall ΔVE (repeated VE - current only VE) was -13.6% (95% CI: -33.2%, 3.2%), indicating lower VE among repeatedly vaccinated children.

Current influenza vaccination programs provide substantial protection, but could be further improved by strategies that mitigate repeat vaccination effects, and further research is needed to identify such strategies.