Childhood Infection and Immunity Longitudinal Cohort

This prospective observational cohort study combines scheduled longitudinal assessments, active surveillance of respiratory infections, healthcare data linkage, and biospecimen-based laboratory analyses to characterize biomarkers of anti-infective immunity in children and assess their risk of infection. Questionnaire data and biological samples will be collected at baseline and annual visits. Respiratory infection events will be monitored through regular contact with participants' families, voluntary symptom reporting, sentinel hospital surveillance, and available healthcare records. Remote follow-up will generally occur every 8 to 12 weeks and may increase to every 4 weeks during periods of increased respiratory pathogen activity. Eligible respiratory infection events will trigger acute-phase sampling, preferably within 3 days after illness onset or at the initial clinical visit, and recovery-phase sampling at 21 ± 4 days after onset. All eligible respiratory infection-related hospitalizations will trigger sampling. Outpatient or emergency department events may trigger sampling no more than twice per participant per year, with an interval of at least 6 weeks between events. Laboratory assessments may include vaccine- and pathogen-specific antibodies, respiratory pathogen detection, cytokines, gut microbiome analysis, genomic analysis, and selected single-cell analyses.