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

This is a prospective, observational, multicenter study employing continuous, convenience sampling, stratified by study site and by fortnightly interval, enrolling until the target sample sizes are achieved over the first epidemiological season (2026) and the second epidemiological season (2027). The study population includes children aged 0 to 24 months presenting to the outpatient or inpatient facilities of the study sites with signs and symptoms of acute respiratory infection (ARI). In Vietnam there is no single guideline for diagnosing ARI, but rather there are guidelines specific to clinical conditions such as pharyngitis, rhinitis, tonsilitis, or pneumonia. This study will use an operational definition where ARI is defined as the presence of one or more of the following clinical features: cough, rhinorrhea, sore throat, dyspnea, and / or fever. RSV testing using nasopharyngeal (NP) swabs will be collected from all eligible children with consent to participate in the study; if a NP swab is not feasible, a mid-turbinate nasal swab will be used. Specimens will be transported to a central laboratory for real-time, reverse transcription-polymerase chain reaction (RT-PCR) testing. Furthermore, the stored viral transport media from all the RSV-positive samples will be used for whole genome sequencing (WGS) to determine the subgroup (A or B) of the circulating RSV strains and to analyze the genotype. The parent or legally acceptable representative (LAR) of pediatric patients will provide information regarding the patient's demography, such as the family's socioeconomic status and household environment (e.g., exposure to tobacco smoke, overcrowding). The children's immunization history and their mothers' immunization history (i.e., vaccines such as influenza and Tdap received during the pregnancy as part of a maternal immunization program) will be established, which is not to estimate vaccine effectiveness but to document the level of vaccine uptake in these populations during the study. Children with maternal RSV vaccination will be excluded. Crucially, the parent or LAR also will report on the child's complete medical history, including perinatal / pregnancy history, presence of congenital anomaly, previous history of illness, and detailed history and clinical course of the current illness episode. Maternal hypertension and history of immunological factors will not be explored. Data on the medical treatment will be abstracted from the patients' medical records. One month after the outpatient clinic visit or hospital discharge, the parent or LAR will be contacted via mobile phone to ascertain the children's current health status and to collect information on the family's direct costs due to the illness.

Inclusion Criteria: * Children aged 0 to \