Durability of neutralizing RSV antibodies following nirsevimab administration and elicitation of the natural immune response to RSV infection in infants

Nirsevimab is an extended half-life monoclonal antibody specific for the prefusion conformation of the respiratory syncytial virus (RSV) F protein, which has been studied in preterm and full-term infants in the phase 2b and phase 3 MELODY trials. We analyzed serum samples collected from 2,143 infants during these studies to characterize baseline levels of RSV-specific immunoglobulin G antibodies and neutralizing antibodies (NAbs), duration of RSV NAb levels following nirsevimab administration, the risk of RSV exposure during the first year of life and the infant's adaptive immune response to RSV following nirsevimab administration. Baseline RSV antibody levels varied widely; consistent with reports that maternal antibodies are transferred late in the third trimester, preterm infants had lower baseline RSV antibody levels than full-term infants. Nirsevimab recipients had RSV NAb levels >140-fold higher than baseline at day 31 and remained >50-fold higher at day 151 and >7-fold higher at day 361. Similar seroresponse rates to the postfusion form of RSV F protein in nirsevimab recipients (68-69%) compared with placebo recipients (63-70%; not statistically significant) suggest that while nirsevimab protects from RSV disease, it still allows an active immune response. In summary, nirsevimab provided sustained, high levels of NAb throughout an infant's first RSV season and prevented RSV disease while allowing the development of an immune response to RSV. The prolonged persistence at elevated levels of nirsevimab, an RSV-specific monoclonal antibody, likely accounts for the observed protection from severe disease throughout an RSV season, while it does not prevent the induction of a natural immune response in RSV-exposed infants.

Automated summary

Nirsevimab is an extended half-life monoclonal antibody that provides sustained, high levels of neutralizing antibodies (NAbs) against the respiratory syncytial virus (RSV) while allowing the development of an immune response. It effectively protects infants from RSV disease throughout their first RSV season and has similar seroresponse rates compared with placebo recipients. It is likely due to the prolonged persistence of nirsevimab at elevated levels that leads to this observed protection.