As several pharma juggernauts prepare for the upcoming respiratory syncytial virus (RSV) season, the FDA has blessed Merck’s entrant.
The monoclonal antibody, clesrovimab, will be known commercially as Enflonsia and will challenge Sanofi and AstraZeneca’s blockbuster Beyfortus.
Enflonsia has been approved to prevent lower respiratory tract disease from RSV in children who are born during or entering their first RSV season, the U.S. regulator said in a social media post. The nod arrived one day ahead of the agency's target decision date.
In the U.S., susceptibility to RSV infections typically begins in November and lasts through April. While symptoms of an RSV infection are often like those of a severe cold, the virus can be deadly for infants, the elderly or those of any age who are immunocompromised.
The Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices is scheduled to meet June 25-27 to formulate recommendations for the use of Enflonsia and shots for several other viruses. In a Monday press release, Merck said it expects to be able to deliver shipments before the start of the next RSV season.
GSK’s Arexvy and Pfizer’s Abrysvo are the top-selling RSV vaccines for older adults and will be joined in the next RSV season by Moderna’s recently approved mRESVIA.
Meanwhile, Sanofi and AZ have been unchallenged with Beyfortus as the lone prophylactic RSV antibody for children. The drug generated sales of 1.7 billion euros ($1.8 billion) last year. In 2023, reports emerged that demand for the antibody outstripped early supply.
Also Monday, Sanofi said that it and partner AZ have already manufactured as many doses of Beyfortus as they supplied last year and that production remains ongoing. The companies added that they will begin shipping early in the third quarter.
Enflonsia has delivered trial results that suggest it can compete with Beyfortus. Eight months ago, Merck revealed that clesrovimab reduced RSV-related hospitalization by 84% compared to placebo through five months, meeting the secondary endpoint in a late-stage study.
The trial—which included preterm and full-term babies up to a year old—also achieved its primary endpoint, reducing the incidence of RSV-associated medically attended lower respiratory infections (MALRI) requiring at least one indicator of lower respiratory infection by 60% versus placebo.
Because of the differences in their respective trials, it is difficult to draw direct conclusions between Enflonsia and Beyfortus. While both are monoclonal antibodies, they target different antigenic sites, complicating any comparison.
One potential advantage for clesrovimab is its single dose as opposed to two different dosing options for Beyfortus depending on the weight of the baby. Merck also has claimed that the virus is more likely to develop resistance to Sanofi and AZ’s shot.
“Enflonsia combines dosing convenience with strong clinical data showing significant reductions in RSV disease incidence and hospitalizations, making it a promising new intervention to help protect infants from RSV,” Octavio Ramilo, M.D., of the St. Jude Children’s Research Hospital and an investigator for two clesrovimab trials, said in a release.
Meanwhile, Sanofi has pointed to Beyfortus’ 75% reduction in the incidence of MALRI versus Enflosia’s figure of 60%. The French company also has emphasized Beyfortus’ duration of protection through six months, which is now reflected on its new label in Europe.
Another competitor in the infant RSV market is Pfizer’s Abrysvo, which can be given to women in weeks 32 through 36 of their pregnancies to protect their newborns. The advantage of Abrysvo that could eventually spark more widespread use is its price. As a vaccine, it can be produced more inexpensively than a monoclonal antibody.
In the U.S., there are roughly 58,000 children under age 5 who are hospitalized annually with RSV infections, with between 100 to 500 deaths per year in the age group. In low- and middle-income nations, RSV is the second-leading cause of death after malaria.
Globally, there are roughly 3 million children younger than age 5 who are hospitalized with the infection each year and 100,000 who die.