Single dose vaccination among infants and toddlers provides modest protection against influenza illness which wanes after 5 months.

In their first season of vaccination, young children are recommended 2 doses of influenza vaccine, but a two-dose schedule might be difficult to implement in many countries. Within a cohort study of 742 children aged 6 to <24 months in Managua, Nicaragua, this study estimated effectiveness of partial vaccination from 3 to 9 months post-vaccination. Vaccine effectiveness (VE) was 74% (95% CI: 24%, 91%) within 3 months and 55% (95% CI: 10%, 77%) within 4 months. There was not significant protection beyond 5 months. Partial vaccination might confer some benefits but should be followed by a second dose.