Nirsevimab, a long-acting monoclonal antibody against respiratory syncytial virus (RSV), reduced medically attended RSV lower respiratory tract infection by 74.5% in healthy late-preterm and term infants entering their first RSV season, according to the phase 3 MELODY trial published in The New England Journal of Medicine. The finding, from 1,490 infants randomized 2:1 to nirsevimab or placebo, provided the pivotal evidence for the FDA’s July 2023 approval of nirsevimab (Beyfortus), the first agent recommended for RSV prevention in all infants regardless of underlying risk.
Why RSV has needed a broad prevention tool
Respiratory syncytial virus (RSV) is the leading cause of hospitalization for infants under the age of one in the United States, with hospitalization rates roughly 16 times those of influenza. Each year, an estimated 590,000 RSV disease cases in infants under one require medical attention. Until 2023, the only available preventive was palivizumab, a shorter-acting monoclonal antibody restricted to a narrow high-risk population (extremely preterm infants and those with congenital heart or lung disease), requiring five monthly doses per season. Most infants hospitalized for RSV, however, are otherwise healthy full-term babies, meaning the great majority of severe RSV disease occurred in a population with no available preventive.
How nirsevimab works
Nirsevimab is a recombinant human immunoglobulin G1 kappa monoclonal antibody that binds the prefusion form of the RSV F protein, the glycoprotein RSV uses to enter respiratory cells. By locking F in its prefusion conformation, nirsevimab prevents membrane fusion and viral entry. The antibody has been engineered with a triple amino acid substitution (YTE) in its Fc region that markedly extends its half-life, allowing a single intramuscular dose to provide protection through an entire five-month RSV season. This differs fundamentally from a vaccine: nirsevimab supplies ready-made antibody, providing passive immunity from the day of administration rather than relying on the infant’s own immune system to mount a response.
How the trial was designed
MELODY was a phase 3, multicenter, randomized, double-blind, placebo-controlled trial. Investigators enrolled healthy infants born at a gestational age of at least 35 weeks who were entering their first RSV season. Participants were randomized 2:1 to receive a single intramuscular injection of nirsevimab or placebo, with nirsevimab dosed by weight (50 mg for infants under 5 kg, 100 mg for those weighing 5 kg or more). Enrollment began in July 2019 and was suspended after the WHO declared the COVID-19 pandemic on 11 March 2020. The primary analysis of the initial 1,490 participants was published in NEJM in 2022, with subsequent analyses of the fully enrolled MELODY cohort (a total of approximately 2,550 participants) published in follow-up NEJM letters.
The primary endpoint was the incidence of medically attended RSV-associated lower respiratory tract infection over 150 days post-dose, with cases requiring both predefined clinical severity criteria and real-time reverse-transcriptase PCR confirmation. Key secondary endpoints included hospitalization for RSV lower respiratory tract infection and very severe RSV lower respiratory tract infection.
What the trial found
The primary endpoint was met with a large effect size. Efficacy against medically attended RSV lower respiratory tract infection was 74.5% (95% CI, 49.6 to 87.1; P<0.001). In the fully enrolled analysis published in 2023, efficacy against hospitalization for RSV lower respiratory tract infection was 76.8% (95% CI, 49.4 to 89.4), with similar efficacy observed against very severe medically attended RSV lower respiratory tract infection. Serious adverse events were similar between the nirsevimab and placebo groups, with no clinically meaningful safety differences observed.
Pooled analyses combining MELODY with earlier phase 2b data in preterm and term infants born at 29 weeks gestation or later found efficacy of 79.5% (95% CI, 65.9 to 87.7) against medically attended RSV lower respiratory tract infection, 77.3% against hospitalization, and similar effect sizes for very severe disease. Nirsevimab has also been shown to have a similar safety profile to palivizumab in higher-risk infants (in the MEDLEY trial), and does not appear to be associated with enhanced disease in a subsequent RSV season.
Real-world evidence has confirmed the trial data
Following the FDA’s July 17, 2023 approval, the CDC’s Advisory Committee on Immunization Practices voted unanimously (10-0) on August 3, 2023 to recommend nirsevimab for all infants under 8 months entering their first RSV season, with additional recommendations for high-risk children in their second season. The 2023-2024 RSV season provided the first real-world test. In the HARMONIE trial in France, Germany, and the United Kingdom, hospitalization for RSV lower respiratory tract infection occurred in 11 of 4,037 infants receiving nirsevimab (0.3%) compared with 60 of 4,020 infants receiving standard care (1.5%), corresponding to real-world efficacy of 83.2% (95% CI, 67.8 to 92.0; P<0.001). A separate Kaiser Permanente Northern California study reported similarly high effectiveness in U.S. clinical practice.
How the effect size should be read
An efficacy of 74% to 83% against medically attended RSV lower respiratory tract infection is a large protective effect for a single-dose intervention. The absolute risk reduction is smaller in absolute terms because RSV lower respiratory tract infection, while common, does not affect every infant each season. Even so, given the scale of infant RSV disease (roughly 590,000 U.S. medical encounters annually and RSV’s standing as the leading cause of infant hospitalization), a 74% relative reduction translates to a substantial population-level effect. Public health modeling suggests that widespread nirsevimab use could meaningfully reduce winter surges of RSV disease that strain pediatric hospitals.
What the evidence cannot yet answer
MELODY was designed to detect efficacy against medically attended lower respiratory tract infection, not hospitalization or mortality directly. Longer follow-up will be needed to determine whether nirsevimab reduces later-life wheezing, asthma, or other post-RSV respiratory sequelae that some observational data suggest are linked to early-life severe RSV disease. Global equity in access remains a substantial concern: nirsevimab’s cost limits deployment in low-income settings where the disease burden is highest. And whether repeat dosing will be needed for children remaining at high risk into subsequent seasons, and how nirsevimab compares with newer competitors such as clesrovimab (approved in 2025), remain to be established.
What the approval settled
The FDA’s approval and the CDC’s universal recommendation together represent a shift in how infant RSV is prevented. Instead of hoping a baby avoids RSV until they are older and sturdier, protection becomes a planned, one-shot decision that parents can time to birth or to the start of RSV season. Public health teams can now aim for near-universal coverage of infants in a way that was never previously possible. And by demonstrating that a long-acting monoclonal antibody can provide broad, seasonal protection with a single dose, MELODY has provided a template that is already being followed for other pathogens where vaccines have proven difficult to develop.
The trial authors concluded that “nirsevimab prevented medically attended RSV-associated lower respiratory tract infection when administered as a single dose before the RSV season in healthy late-preterm and term infants.” Real-world data from the first full season of use have been consistent with that finding, providing the confirmatory evidence that has changed how pediatricians, obstetricians, and families prepare for winter.
References
- Hammitt LL, Dagan R, Yuan Y, et al; MELODY Study Group. Nirsevimab for prevention of RSV in healthy late-preterm and term infants. New England Journal of Medicine, 2022; 386: 837-846. DOI: 10.1056/NEJMoa2110275
- Drysdale SB, Cathie K, Flamein F, et al. Nirsevimab for prevention of hospitalizations due to RSV in infants. New England Journal of Medicine, 2023; 389: 2425-2435. DOI: 10.1056/NEJMoa2309189