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.
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.
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.
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).
Other
Cross-sectional
Not listed
Proportion of ARI that is RSV positive
Frequencies, percentages of RSV positivity
The seasonal patterns of RSV infection
Time-series analysis of infection RSV will be performed to assess seasonal trends
Clinical burden of infection RSV
Frequencies, percentages of hospitalization among infection RSV
Whole-genome sequencing results
Frequencies, percentages will be used to describe RSV subgroup (A or B) and genotyping
Direct medical costs
Mean and standard deviations will be used to describe the direct medical costs
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