Hemagglutination inhibition, microneutralization, and neuraminidase inhibition antibody geometric mean titers (with 95% confidence intervals), by infection status and study day. Mild-to-moderate influenza disease (MMID) was defined as 1 or more positive polymerase chain reaction (PCR) tests and meeting the symptom threshold at any point during the challenge period. The gray dotted line displays the trajectory of the combined qualitative reverse-transcription PCR (RT-PCR)–positive asymptomatic and RT-PCR–negative groups (ie, participants who did not meet the definition of MMID).