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.

This is a prospective, observational, multicenter study employing continuous, convenience sampling until the target sample size is achieved (N=2,000). (In this field investigation, healthcare workers will look to enroll potentially eligible patients presenting in their outpatient clinic or inpatient hospital setting. The study population includes children aged 0 to 24 months presenting to the outpatient or inpatient facilities of the study sites with symptoms of acute respiratory infection (ARI).

Participants will receive treatment for their acute respiratory infections strictly in accordance with the standard of care guidelines at the respective research sites. The study's design is a low-interventional study. Infants and children eligible for inclusion in the study and whose parents or LAR have provided consent will have a low intervention nasopharyngeal swab for RSV testing. Patients will also be tested for influenza as per site standard of care and per investigator's judgement. There will be no changes to the patient's treatment regimen or clinical care during their outpatient visit or hospitalization.

The study will be carried out over a period of 24 months, from May 2026 to May 2028, to capture seasonal variations in RSV proportion in two rainy seasons.

The study population will include children aged 0 to 24 months presenting with acute respiratory infection (ARI) to the outpatient or inpatient facilities of the three selected study sites: Dĩ An Health Center, Ba Ria General Hospital, and Lê Văn Thịnh Hospital (all in Ho Chi Minh City).

During the physical examination of pediatric patients presenting at the study sites, the attending physician will notify the study team if any child aged 0-24 months meets the ARI case definition. The study team will then approach the parent or LAR to introduce the team and the study's purpose, and to perform a preliminary assessment of the patient's eligibility.

For infants and children seen in an outpatient clinic and who have an assessment for ARI, the parent or LAR will be approached during the clinic visit. For infants and children who require hospitalization, the parent or LAR will be approached within 24 hours of ARI assessment.

1. Enrollment and Eligibility Verification From the Informed Consent Form (ICF), investigators will collect essential information through a direct interview with the parent or LAR. The study's inclusion and exclusion criteria will be assessed based on the data gathered from both the interview process and the medical records. Eligible patients will then be officially enrolled and assigned a unique Subject ID Code.

2. Sample Collection and Processing Following enrollment, a nasopharyngeal swab will be collected from the participant by a trained research nurse as soon as possible, but within 24 hours of ICF signing. The sample will be collected using a sterile nylon flocked swab and immediately placed into a tube containing 3 ml of viral transport medium (VTM).

   In cases where the attending physician suspects influenza (e.g., during regional outbreaks or based on clinical signs), 0.1 mL of the viral transport medium (VTM) will be immediately aliquoted at the study site for a rapid influenza antigen test. This approach eliminates the need for a second NPS, thereby reducing patient discomfort. Furthermore, this point-of-care test result is intended solely for clinical management and is not included in the primary study data. The remaining 2.9 mL of VTM will be stored and transported at 2-8°C to the site labs within 4 hours of collection for storage at -700C or lower.

3. Laboratory Testing and Analysis All stored nasopharyngeal samples will be sent to the central lab for RSV testing (using qualitative real-time polymerase chain reaction (RT-PCR)). The samples that test positive for RSV will be further utilized for whole genome sequencing (WGS) to determine the subgroup and genotype of the isolated RSV strains.

4. Interviews During outpatient or inpatient treatment at the research sites, investigators will collect relevant information, including demographics, medical history, household environment, lifestyle habits, socioeconomic characteristics, and other epidemiological characteristics, through direct interviews with parents and/or the LAR. Additionally, demographic information about the primary caregivers of participants will be collected. Besides, economic burden data will be collected and categorized as follows: Outpatient care and hospitalization service fees 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 <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.

The study population includes children aged 0 to 24 months presenting to the outpatient or inpatient facilities of the study sites with symptoms of acute respiratory infection (ARI).