Antibody titers measured by hemagglutination inhibition (HAI) correlate with protection against influenza virus infection, and are used to specify criteria for vaccine licensure. In a randomized controlled trial of seasonal influenza vaccination in 773 children 6-17y, we estimated that HAI titers of 1:40 against A(H1N1)pdm09 and B(Victoria-lineage) were associated with 48% (95% confidence interval, CI: 30%-62%) and 55% (95%CI: 32%-70%) protection against PCR-confirmed infection with each strain. Our analysis accounted for waning in antibody titers over time, and could be particularly useful in settings where influenza activity is delayed or prolonged relative to measurement of antibody titers.