Effectiveness of Partial and Full Influenza Vaccination Among Children Aged <9 Years in Hong Kong, 2011-2019

Background. Two doses of influenza vaccination are recommended for previously unvaccinated children aged <9 years, and receipt of 1 dose is sometimes termed partial vaccination. We assessed the effectiveness of partial and full influenza vaccination in preventing influenza-associated hospitalization 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 from September 2011 through March 2019. Vaccination and influenza status were recorded. Fully vaccinated children included those vaccinated with 2 doses or, if previously vaccinated, those vaccinated with 1 dose. Partially vaccinated children included those who should have received 2 doses but only received 1 dose. We estimated vaccine effectiveness (VE) by using conditional logistic regression models matched on epidemiological week. Results. Overall VE estimates among fully and partially vaccinated children were 73% (95% confidence interval, 69%-77%) and 31% (95% confidence interval, 8%-48%), respectively. A consistently higher VE was observed in children fully vaccinated against each influenza virus type/subtype. The effectiveness of partial vaccination did not vary by age group. Conclusions. Partial vaccination was significantly less effective than full vaccination. Our study supports the current recommendation of 2 doses of influenza vaccination in previously unvaccinated children <9 years of age.