Effectiveness of inactivated influenza vaccine in children aged 9 months to 3 years: an observational cohort study

Background Few prospectively collected data are available to support the effectiveness of inactivated influenza vaccines in children younger than 2 years. We aimed to establish the effectiveness of trivalent inactivated influenza vaccine against laboratory-confirmed influenza A and B infections in a cohort of children younger than 3 years. Methods In a prospective cohort study during the influenza season of 2007-08 in Turku, Finland, between Jan 14 and April 9, 2008, we assessed the effectiveness of trivalent inactivated influenza vaccine against laboratory-confirmed influenza A and B infections in children aged 9 months to 3 years. Our study was part of a clinical trial on antiviral treatment of influenza in children (ClinicalTrials.gov, number NCT00593502). The vaccine was given as part of the Finnish vaccination programme as a 0.5 mL injection. Children enrolled into our study through mailed announcements and local advertisements were examined every time they had fever or signs of respiratory infection. No exclusion criteria were used for enrolment. Influenza was diagnosed with viral culture, antigen detection, and RT-PCR assays of nasal swab specimens. Vaccination status of children was determined by parental report. We calculated the primary effectiveness of influenza vaccination by comparing the proportions of infections in fully vaccinated and unvaccinated children in the follow-up cohort. Findings We enrolled 631 children into our study with a mean age of 2.13 years (range 9-40 months). Seven (5%) of 154 fully vaccinated children and 61 (13%) of 456 unvaccinated children contracted influenza during the study (effectiveness 66%, 95% CI 29-84; p=0.003). In the subgroup of children younger than 2 years, four (4%) of 96 fully vaccinated children and 21 (12%) of 172 unvaccinated children contracted influenza (66%, 9-88, p=0.03). We were unable to record any adverse events associated with the vaccination of the children in our study. Interpretation Trivalent inactivated influenza vaccine was effective in preventing influenza in young children, including those younger than 2 years. Our findings suggest that influenza vaccine recommendations should be reassessed in most countries.