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.