Seasonal influenza vaccine effectiveness in pre- and full-term children aged 6–23 months over multiple seasons

•We studied influenza vaccine effectiveness among children aged 6–23 months.•Influenza-coded ambulatory visits were the main outcome under investigation.•Only full vaccination was effective, and only among full-term children.•Further analyses suggest the observed benefits were not due to confounding.•Future studies of vaccine effectiveness in preterm children are warranted.

IntroductionThis study aimed to determine the effectiveness of seasonal influenza vaccine in pre- and full-term children aged 6–23 months.MethodsWe examined a cohort of 683,354 young children (7.7% preterm) over five influenza seasons (2004–2005 to 2008–2009) in Ontario, Canada. Vaccine effectiveness was estimated using influenza-coded ambulatory visits during virologically-confirmed influenza season periods as the outcome and multivariable Cox proportional hazards modeling.ResultsFull vaccination was associated with a 19% reduction in influenza-coded ambulatory visits (HR = 0.81; 95% CI, 0.68–0.97) in all children, and an 18% reduction in full-term children (HR = 0.82; 95% CI, 0.68–0.99). We did not find significant vaccine effectiveness for preterm children. No benefit was found for partial vaccination.ConclusionsIn children younger than two years, only full influenza vaccination is associated with reduced influenza-coded ambulatory visits. Since the effectiveness of influenza vaccination in preterm children remains uncertain, further study of this highly vulnerable population is warranted.