Influenza vaccine effectiveness against influenza-associated hospitalization in children in Hong Kong, 2010-2020.

Chua, Huiying; Kwan, Mike Y W; Chan, Eunice L Y; Wong, Joshua S C; Peiris, J S Malik; Cowling, Benjamin J; Chiu, Susan S

BACKGROUND Influenza virus infections can cause hospitalizations in children, and annual vaccination of children can provide protection against influenza. METHODS We analyzed a test-negative design study with data spanning from 2010/11 through 2019/20 to evaluate influenza vaccine effectiveness (VE) against influenza hospitalization in children by age group, influenza type/subtype and time period within each season. We enrolled children admitted to hospital with acute febrile respiratory illnesses. Nasopharyngeal aspirates were tested by culture and/or RT-PCR to determine influenza status, and vaccination status was obtained by interviewing parents or legal guardians and was verified where possible. VE was estimated by conditional logistic regression model adjusting for sex, age and age-squared, matching on week. RESULTS Influenza seasons in Hong Kong are prolonged with influenza-associated hospitalizations occurring in almost every month of the year during the study period. Influenza vaccination was effective in preventing influenza-associated hospitalizations in children of all ages. Influenza VE was higher in younger children than in older children, and higher against hospitalization due to influenza A(H1N1)pdm09 than A(H3N2) and B. CONCLUSIONS The childhood influenza vaccination program in Hong Kong has prevented influenza-associated hospitalizations particularly in younger children. Our findings support the use of influenza vaccines in children as an effective approach to influenza control and prevention.