Objective To assess optimal timing of influenza vaccination in young children.
Design Population based cohort study.
Setting United States.
Participants Commercially insured children aged 2-5 years who were vaccinated against influenza during 2011-18.
Main outcome measure Rates of diagnosis of influenza among children who were vaccinated against influenza, by birth month.
Results Overall, 819 223 children aged 2-5 received influenza vaccination. Children vaccinated in November and December were least likely to have a diagnosis of influenza, a finding that may be confounded by unmeasured factors that influence the timing of vaccination and risk of influenza. Vaccination commonly occurred on days of preventive care visits and during birth months. Children born in October were disproportionately vaccinated in October and were, on average, vaccinated later than children born in August and earlier than those born in December. Children born in October had the lowest rate of influenza