Lower respiratory tract infections following respiratory syncytial virus monoclonal antibody nirsevimab immunization versus placebo: Analysis from a Phase 3 randomized clinical trial (MELODY).

Background: Nirsevimab is an extended half-life, highly potent neutralizing monoclonal antibody against the respiratory syncytial virus (RSV) fusion protein, with efficacy in preventing RSV-associated medically attended (MA) lower respiratory tract infection (LRTI) in infants and medically vulnerable children (aged ≤24 months). This post-hoc exploratory analysis examined the incidence of LRTI from RSV and other respiratory pathogens during a 2:1 randomized, double-blind, placebo-controlled, phase 3 study of nirsevimab, in healthy-term and late-preterm (i.e. gestational age ≥35 weeks) infants entering their first RSV season (MELODY).

Methods: 3012 participants were randomized to nirsevimab (n = 2009) or placebo (n = 1003). Nasopharyngeal swabs were collected from infants presenting with an LRTI and tested for 22 different respiratory pathogens using the BioFire® Respiratory 2.1 Panel. Incidence of RSV and non-RSV MA-LRTIs through Day 511 and LRTI severity per the ReSViNET scale were assessed.

Results: 852 nasopharyngeal swabs were collected from 561 participants through Day 511: 519 swabs from 337 nirsevimab participants and 333 swabs from 224 placebo participants. RSV and non-RSV infections were detected in 193/852 (22.7%) and 551/852 (64.7%) swabs, respectively. RSV infection rates were lower with nirsevimab compared with placebo, including RSV-rhinovirus/enterovirus coinfections. Rates of other viral infections were similar between study arms. Approximately 70% of single RSV infections and RSV coinfections were adjudicated as mild, and 26.2% of single RSV infections and 24.5% of RSV coinfections required hospitalization.

Conclusions: Nirsevimab protected against RSV single and coinfections, with no evidence of replacement of RSV with other respiratory viruses. Background: ClinicalTrials.gov NCT03979313.