MSD’s Enflonsia (clesrovimab) has received FDA approval for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease (LRTD) in newborns and infants born during or entering their first RSV season. Enflonsia is a long-acting monoclonal antibody (mAb) that can provide protection through a typical five-month RSV season with a 105mg dose, regardless of weight. The recent approval of Enflonsia was based on clinical trial results from the Phase IIb/III CLEVER trial (NCT04767373) evaluating a single dose of Enflonsia administered to preterm and full-term infants. Treatment with Enflonsia reduced the incidence of RSV-associated medically attended lower respiratory infections through 5 months by 60.5% compared to placebo, and RSV-related hospitalisations through 5 months by 84.3% compared to placebo.
Enflonsia will now compete with Sanofi and AstraZeneca’s mAb Beyfortus (nirsevimab) for patient shares in the US. Beyfortus received FDA approval for the prevention of RSV LRTD in young children in July 2023 and has gained strong traction in the market. The drug achieved sales of over $1 billion in the US alone in 2024.
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The benefit of Enflonsia is that it is the first and only RSV preventative option for the pediatric market that does not require weight-based dosing, which makes administration easier and more convenient. Both Beyfortus and Enflonsia have strong efficacy and safety profiles. Beyfortus and Enflonsia are expected to completely replace the use of Sobi’s Synagis (palivizumab), an older mAb which requires monthly dosing throughout the RSV season and is associated with more toxicities, with side effects including fevers and rashes, amongst others.
MSD plans to make Enflonsia available for the 2025–26 RSV season. Enflonsia is expected to become a successful drug in the pediatric market, reaching sales of $488 million in the US, and $892 million globally by 2031 according to GlobalData’s analyst consensus forecast. Nonetheless, GlobalData still expects Beyfortus to perform better commercially than Enflonsia over this time period since it is an earlier-market entrant with a similar safety and efficacy profile. Furthermore, unlike Enflonsia, Beyfortus is also approved for use in children up to 24 months who remain vulnerable to RSV through their second RSV season.
The CDC’s Advisory Committee on Immunization Practices (ACIP) meeting is due to be held on June 25–27, when recommendations for RSV immunisations will be made. These recommendations are likely to heavily influence Enflonsia’s ability to capture market share from Beyfortus. With RSV being the leading cause of infant hospitalisation in the US, Enflonsia provides an alternative, convenient and efficacious option to prevent RSV LRTD in neonates and infants.
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