Respiratory syncytial virus (RSV) is a leading cause of severe lower respiratory tract infection in young children, and a substantial proportion of severe cases occur in previously healthy infants. The gut-lung axis suggests that gut microbiome composition may modulate respiratory immune responses. This prospective observational study in Vietnam will compare gut microbiome profiles and systemic immune cytokine responses between infants with severe RSV infection and those with mild RSV infection, aiming to identify microbiome-immune signatures associated with disease severity.

This is a single-center, prospective observational study conducted at Vietnam National Children's Hospital in Hanoi over 48 months. Approximately 250 infants aged 1-24 months with RT-PCR-confirmed RSV lower respiratory tract infection will be enrolled and classified into severe vs mild groups based on need for advanced respiratory support (HFNC/CPAP/invasive ventilation) and/or PICU admission versus no/low-flow oxygen requirement. Stool samples collected within the first 24 hours of admission will undergo 16S rRNA sequencing (V3-V4 region) to characterize gut microbiome diversity and taxa abundance. Blood samples collected within the first 24 hours will be used to quantify key cytokines (e.g., TNF-α, IL-6, IL-8, IL-1β, IFN-γ, IL-10, IL-17A, IL-22) using ELISA; immune cell subsets may be assessed by flow cytometry. Clinical severity will be assessed using standardized pediatric scores (PRISM III, PELOD, pSOFA) and treatment outcomes will be recorded. The study will evaluate associations among microbiome features, immune response markers, and RSV severity to propose candidate integrated biomarkers for early risk stratification.

Inclusion Criteria:

Age 1 to 24 months RT-PCR positive for RSV from respiratory specimen Symptoms consistent with acute lower respiratory tract infection Parent/legal guardian provides informed consent

Exclusion Criteria:

Prematurity <32 weeks gestation or birthweight <1500 g Chronic conditions (e.g., congenital heart disease, chronic lung disease, chronic liver/kidney disease) Primary or acquired immunodeficiency Severe malnutrition (weight-for-age Z-score < -3 SD) Antibiotic use within 2 weeks before admission Probiotic use within 4 weeks before admission Co-infection with other pathogens (viral/bacterial) Stool sample not obtained within 24 hours of admission

Infants with acute lower respiratory tract infection and RT-PCR-confirmed RSV presenting to Vietnam National Children's Hospital