Effectiveness of partial and full influenza vaccination in children aged <9 years in Hong Kong, 2011-2019.

INTRODUCTION: Two doses of influenza vaccination are recommended for previously unvaccinated children aged <9 years, and receipt of one dose is sometimes termed "partial vaccination". We assessed the vaccine effectiveness (VE) against influenza hospitalization of partial and full influenza vaccination among children in Hong Kong.

METHODS: Using the test-negative design we enrolled 23,187 children aged <9 years admitted to hospitals with acute respiratory illness between September 2011 through to March 2019. Vaccination and influenza status were recorded. Fully vaccinated children included those vaccinated with two doses, or one dose if they were previously vaccinated. Partially vaccinated children included those who should receive two doses but received only one dose. We estimated VE using conditional logistic regression models matching on epidemiological week.

RESULTS: Overall VE estimates among fully and partially vaccinated children were 73% (95% confidence interval, CI: 69% ,77%) and 31% (95% CI: 8%, 48%), respectively. Consistently higher VE was observed in fully vaccinated children against each influenza type/subtype. VE of partial vaccination did not vary by age groups.

CONCLUSIONS: Partial vaccination was significantly less effective than full vaccination. Our study supports the current recommendation of two doses of influenza vaccination in previously unvaccinated children <9 years of age.