A single dose of the monoclonal antibody nirsevimab reduced hospitalizations due to respiratory syncytial virus (RSV)-associated lower respiratory tract infections (LRTIs) by 82.7% (95% confidence interval = 67.8%-91.5%; P <.0001) over 180 days in infants during their first RSV season, according to a phase 3 randomized controlled trial conducted in France, Germany, and the United Kingdom.
The study enrolled 8,057 infants aged ≤12 months, randomly assigned to receive either a single intramuscular dose of nirsevimab (n = 4038) or no RSV prophylaxis (standard care; n=4019). Hospitalization occurred in 12 infants (0.3%) in the nirsevimab group compared with 68 infants (1.7%) in the standard care group. The median time to RSV-related hospitalization was 28.5 days (interquartile range [IQR], 5.5–52.0) and 27.0 days (IQR = 13.5–55.5), respectively.
The study also demonstrated 75.3% efficacy (95% confidence interval [CI], 38.1%-91.8%; P = .0013) in preventing very severe RSV-associated LRTI, defined as hospitalization with oxygen saturation <90% requiring oxygen therapy. Additionally, nirsevimab reduced hospitalizations from all-cause LRTIs by 41.9% (95% CI = 23.1%-56.3%; P <.0001).
Efficacy was consistent across age groups (≤3 months, 3–6 months, >6 months), weight categories (<5 kg vs ≥5 kg), and timing of administration (before vs during RSV season). Among neonates aged 0–28 days, 946 received nirsevimab and 963 received standard care.
By country, efficacy was 86.1% in France, 85.9% in the United Kingdom, and 74.4% in Germany. The study included 235 sites across primary and secondary care.
Nirsevimab was well tolerated, with no new safety concerns identified. Grade 1 adverse events occurred in 2,759 infants (68.7%)in the intervention group and 2696 (67.1%) in the control group. Grade 2 events occurred in 1,447 (36.0%) and 1,436 (35.7%) infants, respectively. The most frequently reported events were nasopharyngitis, ear infections, and bronchiolitis.
Eligible participants were healthy infants born at 29 weeks' gestation or later and not considered candidates for palivizumab. Weight-based dosing was used: 50 mg for infants <5 kg and 100 mg for those ≥5 kg.
Saul N. Faust, MD, PhD, of the University Hospital Southampton NHS Foundation Trust, stated: “Nirsevimab offers consistent and sustained protection against hospitalisation due to RSV-associated lower respiratory tract infection for at least 6 months.”
Safety follow-up continues for UK participants through 24 months to evaluate longer-term outcomes, including recurrent wheezing. Researchers noted that the extended protection allows flexibility in aligning administration with routine well-baby visits.
The trial was funded by Sanofi and AstraZeneca and is registered at ClinicalTrials.gov (NCT05437510).
Full disclosures can be found in the published study.
