BACKGROUND: Approaches for determining whether influenza vaccination prevents infection, attenuates illness, or both are important for developing improved vaccines. We estimated influenza infection incidence and evaluated symptom ascertainment methodologies in children to inform future vaccine trial design. METHODS: We conducted a prospective cohort study among children aged 6 to 23 months from May to October 2022. Study nurses collected symptom and temperature data and midturbinate nasal swabs twice weekly irrespective of symptoms
caregivers entered symptom data daily and collected nasal swabs weekly. Samples were tested for influenza with polymerase chain reaction. RESULTS: Of 230 healthy screened children, 93 were enrolled, of whom 87 (94%) completed 6-month follow-up. In total, 95% (4245/4476) of scheduled nurses, 90% (2045/2276) of caregiver swabs, 99% (92/93) of baseline blood collections, and 67% (9245/13 768) of scheduled symptom diaries were completed. Polymerase chain reaction-confirmed influenza incidence was 65% (60/93) for ≥1 infection
11 (18%) individuals had 2 episodes and 1 (2%) had 3. Of 73 episodes, 55 (75%) had ≥1 symptom and 37 (51%) had fever (measured and/or reported). Median infection duration was 7 days (IQR, 4-9). Human RNase P gene was detected in 99% (2032/2045) of caregiver-collected swabs, through which 5 additional episodes were identified. Per episode, caregivers' diaries of reported and measured fever were 19% (25/73, 34%) and 11% (15/73, 21%) higher than nurse-reported (11/73, 15%) and nurse-measured (7/73, 10%) fever, respectively. CONCLUSIONS: The incidence of influenza infection was high and mainly symptomatic, suggesting that this platform could be suitable for future trials of vaccine efficacy and correlates of protection against infection and illness in children.