A household cluster of RSV infection causing severe pneumonia in a four-month-old infant

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection in infants. We analyzed a household RSV cluster of severe RSV infection to inform targeted interventions and outcome evaluation. Clinical and epidemiological data were collected for characteristic analysis. Species of RSV positivity was confirmed by RT-qPCR; G gene and whole-genome sequencing were performed for phylogenetic and mutation analyses. A household cluster affected four family members. In order of symptoms, the first case (Case A, 31-year-old postpartum woman) was diagnosed with pneumonia, the second case (Case B, a four-month-old male infant of Case A) developed severe bronchiolitis with hospitalization for 7 days, and receiving nebulization for 30 days. Final cases (Cases C and D, father and babysitter of Case B) developed mild upper respiratory tract infections. The period of transmission from first to final cases spanned 17 days. During hospitalization, Case B showed marked immune changes: CD3 + , CD4 + naive, and CD4 + T lymphocytes were decreased by approximately 3-fold and 3.3-fold, respectively, while CD8 + , CD8 + naive, CD8 + central memory (CM), and CD4 + CM T lymphocytes were reduced by 1.5-fold, 1.22-fold, 2.72-fold, and 1.8-fold. During the six-month follow-up, respiratory infections recurred monthly in first three months. The RSV strain of Case B was B.D.E.1 lineage, closely related to US strain (PQ117674.1, 99.76% similarity) and separated from location B.D.E.1 in 2023–2024 and contemporaneous seasons [with 59 nucleotides and 2 amino-acids (A10S, K219I) mutation in G gene]. For RSV infants with severe disease, household members are a potential key infectious source, and genetic mutations could contribute to the exacerbation of clinical symptoms. • Household RSV cluster: involved 4 family members, with 1 infant case of severe bronchiolitis. • Infant immune alterations: γδ T cells halved; CD3+/CD4+/CD8+/CD45+ reduced, CD19+ slightly elevated. • Prolonged transmission: 17-day span from first (postpartum woman) to final (father/babysitter) cases. • RSV B.D.E.1 strain: 99.76% similar to US strain, with 59-nucleotide/2-amino-acid G gene mutations. • Key implications: household contacts as sources; RSV mutations linked to symptom exacerbation.