Clinical outcome of respiratory syncytial virus associated medically attended respiratory tract infection following nirsevimab administration among infants and children

Abstract

Background: Respiratory syncytial virus (RSV) is a leading cause of morbidity and mortality among children under five years. In Thailand, the long-acting monoclonal antibody nirsevimab was approved in 2024 and implemented in 2025 as a passive immunization to reduce RSV-associated lower respiratory tract infections and related hospitalizations.

Objectives: This study aimed to describe the clinical characteristics of children who developed symptomatic RSV infection after receiving nirsevimab, and to identify risk and protective factors associated with RSV infection among those who received the antibody.

Methods: This retrospective cohort study was conducted at King Chulalongkorn Memorial Hospital (KCMH). Children aged ≤12 months who received nirsevimab during the RSV season (May–November 2025) were included. Data were collected through a review of medical records and telephone interviews with parents.

Results: Of 294 infants who received nirsevimab, 180 (61.2%) were enrolled. The median age was 6 months (IQR 2–8), and 51.7% were male; 66.1% were healthy. Seven participants (3.9%) had confirmed RSV infection. The incidence rate of RSV-LRTI requiring hospitalization was 1.1 per 1000 person-months (95%CI 0.15-7.9), while the incidence rate of symptomatic RSV infection was 7.8 per 1000 person-months (95%CI 3.2-15.9). The median time from nirsevimab administration to infection was 13 weeks (IQR 6–16). RSV exposure was the only significant risk factor (RR 6.4, 95% CI 1.5–26.3; p value <0.05).

Conclusions: Nirsevimab appears to be an effective preventive strategy in both healthy and high-risk infants, with low hospitalization rates and predominantly mild breakthrough infections. RSV exposure remains the key risk factor, underscoring the importance of continued surveillance to optimize its use.