A Potential Platform for Future Vaccine Trials Identifies a High Incidence of Symptomatic and Asymptomatic Influenza Infection Among Children Aged 6 to 23 Months in South Africa

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. In a cohort of 6- to 23-month-old children in South Africa who were swabbed twice weekly irrespective of symptoms, influenza infection incidence was high and mainly symptomatic, suggesting that this platform could be suitable for future trials of vaccine efficacy and correlates of protection.