Abstract
<title>Abstract</title> <p>Background A simultaneous outbreak of respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) occurred in Ube, Yamaguchi Prefecture, Japan, from January to April 2025. Both viruses belong to the family Pneumoviridae and cause clinically similar respiratory illnesses in children. Whether sharing a care room between RSV-infected and hMPV-infected children leads to cross-infection has not been previously investigated. Methods Children aged 11 months to 11 years who tested positive for RSV or hMPV by rapid antigen tests at Suzuki Pediatric Clinic were enrolled in the sick-child care facility of the same clinic. RSV-positive and hMPV-positive children were cared for in the same room. Cross-infection was prospectively monitored for 1 to 4 months after discharge. Results A total of 54 children participated (RSV: 22, hMPV: 32; male: 31, female: 23; median age: 3 years, range: 11 months to 11 years). Shared care was conducted over 23 sessions from January to April 2025, with 2 to 7 children per session in rooms measuring 10.4 to 22.4 m². No cross-infection was observed during the 1- to 3-month follow-up period. One RSV-infected infant (aged 11 months) subsequently developed hMPV infection 21 days after discharge; however, given that this interval exceeds the known incubation period of hMPV (3–6 days), this case was considered a community-acquired infection. Conclusion Shared sick-child care of RSV- and hMPV-infected children did not result in cross-infection over a follow-up period of 1 to 3 months. Viral interference, possibly mediated by the innate immune response including interferon induction, may explain this finding. Shared-room care of children with RSV and hMPV may be a safe and practical option for sick-child care facilities during co-epidemic periods.</p>