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.

Inclusion criteria

• Children from birth through 7 years of age (younger than 8 years at enrollment).
• The participant's parent or legal guardian is able to understand the study, voluntarily agrees to participation, and provides written informed consent.
• The participant resides in the study area and is expected to be available for long-term follow-up.
• For the General Community Cohort: no known serious underlying disease before enrollment.
• For the Infection Observation Cohort: either a history of hospitalization for an infectious disease before 3 years of age, with documented information on the pathogen, infection site, or clinical diagnosis; or at least three healthcare visits for respiratory infections during the 12 months before enrollment.
• For the Childhood Asthma Cohort: physician-confirmed asthma or highly suspected asthma according to the protocol-defined age-specific criteria. Children younger than 1 year will not be included in this cohort.

Exclusion criteria

• An acute illness within 14 days before enrollment, including respiratory, gastrointestinal, dermatologic, or urinary symptoms or conditions specified in the protocol.
• Known primary immunodeficiency, malignant tumor, serious congenital disorder, or another condition that may substantially alter immune status.
• Long-term use of immunosuppressive or other immunomodulatory medications, defined as continuous or intermittent use for a cumulative duration of at least 3 months.
• A diagnosed coagulation abnormality or bleeding disorder, including coagulation factor deficiency, other coagulation disorders, or platelet abnormalities.
• Any other condition that, in the investigator's judgment, makes the child unsuitable for participation.