Beyfortus is a long-acting monoclonal antibody approved for the prevention of RSV lower respiratory tract disease in newborns and infants born during or entering their first RSV season, and for children up to 24 months of age who remain vulnerable to severe RSV disease through their second RSV season.


RT’s Three Key Takeaways:

  1. Highly Effective RSV Prevention — Beyfortus reduced the risk of RSV disease by 87% in infants and led to fewer emergency visits and hospitalizations, showing strong protection across a broad population.
  2. Dramatic Hospitalization Reduction — A post hoc analysis showed a 98% drop in RSV-related hospitalizations among healthy, full-term babies who received Beyfortus compared to those who didn’t.
  3. Robust Real-World and Trial Evidence — Real-world data and prior clinical trials consistently demonstrate Beyfortus’ effectiveness and safety in preventing serious RSV illness in both term and preterm infants.


The largest US real-world study of Beyfortus (nirsevimab-alip) 50mg and 100mg Injection successfully met both of its co-primary endpoints, demonstrating that Beyfortus provides protection for babies against RSV disease, according to a news release from Sanofi.

Data published in Pediatrics further supports the broad use of Beyfortus as a proven RSV prevention designed to protect all infants, including those born healthy or with underlying conditions, full term or preterm, before or during the RSV season, according to a press release from Sanofi.

“These results support Beyfortus as a groundbreaking advancement in infant RSV disease prevention. With more than 40 real-world evidence studies to date, this latest study further strengthens the well-established body of data that shows that Beyfortus provides protection against RSV disease to help safeguard the health of all babies, regardless of whether they are born before or during the RSV season,” said Ayman Chit, head of North America Medical, Vaccines, Sanofi.

The first co-primary endpoint showed babies administered Beyfortus were 87% (CI: 81.7%-91.1%; p<0.0001) less likely to develop RSV disease compared to babies who did not receive Beyfortus. The second co-primary endpoint showed that if a baby who received Beyfortus later tested positive for RSV lung infection there were fewer overall medical visits (adjusted mean difference -0.86, p=0.001) during the illness, including significantly reduced visits to emergency departments and hospitalizations.

Additionally, a post hoc analysis of the study evaluated the effectiveness of Beyfortus in preventing RSV hospitalizations among healthy term babies. Using real-world data from a retrospective cohort design, researchers compared hospitalization rates between babies who received Beyfortus and those who did not. The analysis demonstrated a 98% reduction in RSV hospitalizations among babies given Beyfortus compared to those who were not.

The study was a large, real-world, retrospective cohort analysis that included more than 30,000 healthy, full-term babies who were eligible to receive Beyfortus. The study excluded babies at high risk for severe RSV disease such as those with underlying medical conditions or those born preterm, allowing for a clear evaluation of the impact of Beyfortus in the broader infant population.

The study further builds on the body of US real-world evidence demonstrating the high impact of Beyfortus. A prospective, population-based surveillance study published in JAMA Pediatrics showed a single dose of Beyfortus was 93% (95% CI, 82%-97%) effective in preventing RSV hospitalizations. The study included hospitalized infants under eight months old and accounted for key variables like age, illness onset, and high-risk conditions. Limitations included supply constraints resulting from high demand, a small proportion of hospitalized infants with acute respiratory infection receiving Beyfortus during the study period and potential RSV exposure prior to deployment of Beyfortus among some babies.

Prior to these US real-world studies, Beyfortus was evaluated in two pivotal trials. Beyfortus efficacy and safety were evaluated in two randomized, double-blind, placebo-controlled pivotal trials with the same primary and secondary endpoints versus placebo through 150 days post one dose: incidence of medically attended (includes all healthcare provider visits such as physician’s office, urgent care, emergency room, and hospitalizations) RSV disease, including hospitalization.

Beyfortus significantly reduced the risk of serious RSV disease in term and late preterm healthy infants:

  • In Trial 04, Beyfortus demonstrated a relative risk reduction of 74.9% (95% CI: 50.6, 87.3; p
  • In Trial 03, Beyfortus demonstrated a relative risk reduction of 70.1% (95% CI: 52.3, 81.2; p

In both studies, the secondary endpoint was the incidence of serious RSV disease with hospitalization in term and preterm healthy babies through 150 days post one dose:

  • In Trial 04, Beyfortus demonstrated a relative risk reduction of 60.2% (95% CI: -14.6, 86.2; p=0.09). Primary Cohort: n=1,490 infants ≥35 wGA (placebo: 1.6% [8/496], Beyfortus: 0.6% [6/994])
  • In an exploratory, post hoc analysis of all infants (full study cohort) in Trial 04, Beyfortus demonstrated a relative risk reduction of 76.8% (95% CI: 49.4, 89.4). n=3,012 infants ≥35 wGA (placebo: 2.0% [20/1,003], Beyfortus: 0.4% [9/2,009])
  • In Trial 03, Beyfortus demonstrated a relative risk reduction of 78.4% (95% CI: 51.9, 90.3; p=0.0002). n=1,453 infants ≥29 to

The most common adverse reactions in Trial 04 and Trial 03 were rash (0.9%) and injection site reactions (0.3%).