Figure 1.

Study design and lower respiratory tract illness (LRTI) surveillance decision tree. ^aFor active contacts, site staff contacted the parents approximately every week during the respiratory syncytial virus (RSV) season and every month during interseason periods in countries with seasonal RSV transmission, and approximately every 2 weeks in countries with year-round RSV transmission. RSV seasonality by country is shown in Figure 2, and information on how seasons were determined is included in the Supplementary Materials. ^bFor passive contacts (which occurred throughout the year independently of RSV seasonality), parents contacted the site staff whenever the infant developed (new) symptoms of a respiratory tract illness (RTI), difficulty breathing, or wheezing; if the infant's symptoms worsened; or if there was parental concern (ie, if the parent[s], legally acceptable representative[s], or designate[s] were concerned about the infant's RTI or general health in the context of the RTI and intended to seek medical care). For both active and passive contacts, a protocol-guided phone script was used to ensure all required information was collected. ^cThe decision for scheduling a visit to assess a possible LRTI and procedures during the visit are explained in the “decision tree for LRTI visit.” Assessment visits were conducted by qualified site staff (ie, physicians, nurses, nurse practitioners, physician's assistants) with documented medical training (ie, medical or nursing license). Abbreviations: COVID-19, coronavirus disease 2019; LRTI, lower respiratory tract illness; RR, respiratory rate; RSV, respiratory syncytial virus; SpO₂, blood oxygen saturation measured by pulse oximetry in room air, if feasible.