Epidemiological Characteristics and Clinical Burden of RSV Infection in Children Under 24 Months in Ho Chi Minh City (2026-2028)

Inclusion Criteria: * Children aged 0 to \<24 months (i.e., less than two years old). * Children presenting with a diagnosis of acute respiratory infection (ARI), defined as an illness with one or more of the following symptoms: cough, rhinorrhea, sore throat, dyspnea (including wheezing, tachypnea, or apnea/increased work of breathing), sepsis (for infants aged \<6 months), and/or fever. * Children residing in the lowest administrative division (Phường / Xã) in Vietnam that is designated for the specific study site (outpatient clinic or community hospital). As patients live in the same administrative district as the healthcare facility, this will facilitate follow-up and ensure that the study results are representative of the range of severity of ARI presenting to each specific study site. * Parent or LAR provides written informed consent for their child's participation. Exclusion Criteria: * Children whose mothers received a maternal RSV vaccine during pregnancy or who have themselves received RSV-specific monoclonal antibodies (mAbs). * Children with a known non-respiratory cause for hospitalization. * Children with respiratory symptoms lasting over 14 days. * Children hospitalized elsewhere within the preceding 4 days, excluding admission to the Emergency Department. * Children already enrolled in another RSV study within the preceding 3 months.