Beyfortus now has a direct rival after FDA OK for Enflonsia

MSD will soon enter the market for drugs used to protect infants from respiratory syncytial virus (RSV) infections after getting approval from the FDA for its antibody therapy Enflonsia.

The green light in the US is the first worldwide for Enflonsia (clesrovimab) and makes the long-acting antibody the first competitor to Sanofi and AstraZeneca's blockbuster Beyfortus (nirsevimab), which was cleared by the FDA for a similar indication in 2023.

MSD said that Enflonsia is the first RSV antibody that can be administered as a single 105mg dose regardless of birth weight, alluding to the need for Beyfortus to be given in one weight-adjusted shot. The drug is indicated for the prevention of RSV lower respiratory tract disease in newborns and infants who are born during or entering their first RSV season.

Yesterday, Sanofi said it was accelerating shipments of Beyfortus to get ahead of what it said would be strong demand in the 2025–26 season – and likely with an eye on the entrance into the market of MSD's rival. Beyfortus made $1.8 billion in sales last year, even though supplies were held back by now-resolved production constraints.

Sanofi and AZ, meanwhile, have highlighted new labelling for Beyfortus in Europe that suggests it can provide protection for six months, while MSD's label currently points to five months, although the latter said that is sufficient to cover "a typical RSV season."

Enflonsia will be priced at $556 per dose, according to MSD, which is known as Merck & Co in the US and Canada, and is slated for a commercial launch in July ahead of the 2025-26 RSV season. According to CDC data, Beyfortus also costs $556 for 50mg and 100mg doses.

Will upending the ACIP affect MSD's launch plans?

It's worth noting that MSD's launch plans are dependent on Enflonsia being discussed by the CDC's Advisory Committee on Immunisation Practices (ACIP), which has a meeting scheduled later this month.

The company's announcement came ahead of the shock decision by Health and Human Services (HHS) Secretary Robert F Kennedy Jr to fire all 17 ACIP members, which has caused consternation in public health and industry circles, with the Pharmaceutical Research and Manufacturers of America (PhRMA) industry body saying it "increases uncertainty and vaccine scepticism."

In the US, RSV causes an average of around 58,000 hospitalisations a year, with 100–500 deaths among children younger than five years old. The virus is the second-largest killer of children worldwide after malaria, with deaths disproportionately seen in low-income countries.

Pfizer's RSV vaccine Abrysvo – given to the mother during pregnancy and also approved for use in older adults – is another option for protecting newborns and infants from RSV disease in the first six months of life and costs around $306 for the single required dose.