Mucosal nirsevimab levels in respiratory syncytial virus breakthrough bronchiolitis

A multivariate analysis (appendix pp 1–2) including potential influencing factors (time since nirsevimab injection, age at injection, history of bronchiolitis, ratio of nirsevimab/IgG, breastfeeding, and gestational age at birth) identified only the mucosal ratio of nirsevimab/IgG as statistically significantly associated with RSV infection (coefficient –1·38 [95% CI –2·663 to –0·101], p=0·034). Nirsevimab is administered intramuscularly and distributed through body fluids to the lower respiratory tract, where it blocks RSV entry through direct viral neutralisation.4,5 Mucosal RSV-specific IgA is associated with protection in the upper respiratory tract in animal models,7 low RSV-specific nasal IgA is a risk factor of RSV infection in adults with IgA memory deficiency,8 and IgA mediates recovery during primary RSV infection in young children (