The U.S. Food and Drug Administration (FDA) on Monday approved Merck’s new monoclonal antibody shot, which the company said is designed to protect newborns and infants against respiratory syncytial virus (RSV).
The antibody shot, clesrovimab, will be marketed as Enflonsia and launched this coming RSV season to challenge Sanofi and AstraZeneca’s blockbuster Beyfortus shot.
Beyfortus, first approved in 2023, generated $1.8 billion in sales last year, FiercePharma reported.
PR Newswire projected that the global RSV vaccine and antibody market would reach $13.59 billion by 2030.
Merck said its monoclonal antibody has about the same efficacy rate as Beyfortus. In clinical trials, the shot reduced RSV-related hospitalization by 84.3% compared to placebo, and the incidence of RSV that needed to be medically attended by 60.5% versus placebo, through age 5 months in the clinical trials.
The company also said in a press release that the drug’s safety profile was “generally comparable” to placebo. The most common adverse reactions were injection-site erythema or swelling, and a rash 14 days after the dose, all of which were more common in the vaccine group.
However, the clinical trial results posted on the government website ClinicalTrials.gov included other serious neurological adverse reactions that Dr. Peter Selley, a U.K. general practitioner who has closely followed the development of the drugs for RSV, called “concerning.”
These included febrile convulsions, seizures, facial paralysis and brain injury at about three times the rate in the vaccine group (25 of 2,409) as the placebo group (4 of 1,202).
Children’s Health Defense Senior Research Scientist Karl Jablonowski said the trials showed a serious adverse event rate of 11.71% and included:
- A 50% greater chance of death (three deaths in the group that received the shot versus one in the placebo group).
- A 350% greater chance of an upper respiratory tract infection.
- A 63% greater chance of a lower respiratory tract infection.
- A 41% greater chance of febrile convulsion.
‘More is not better’
Merck touts its shot as superior to Beyfortus because it can be given in a single dose, instead of a two-dose series. Enflonsia is given as a single 105 milligram (mg) dose for all babies. Beyfortus can be administered in doses of 50 or 100 mg, depending on the baby’s weight.
Clesrovimab “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,” Dr. Octavio Ramilo of St. Jude Children’s Research Hospital in Memphis, Tennessee, an investigator in two of the clinical trials, said in the press release.
However, Selley cautioned that “more is not better.” He told The Defender:
“The implications of this are that with only one dose of clesrovimab, tiny preterm babies under 5 kilograms are going to be injected with a relatively much bigger dose of the monoclonal antibody, leading to massive levels of the drug in their circulation.
“In most other fields of medicine, the aim is to find the lowest dose of a drug that is effective, as most side effects are dose-related.”
Jablonowski pointed out that the FDA just approved MenQuadfi, a meningococcal conjugate vaccine with a serious adverse event rate of 5.19% that diminished the immune system and induced RSV infection at a rate 270% greater than the comparator (MENVEO) and febrile convulsions at a rate 426% greater.
“Then the FDA approves Enflonsia, with similar rates of adverse events. If these are the standards for a successful FDA application, what does it take to fail?”
Unclear if CDC will recommend Merck’s new RSV shot
Merck’s Enflonsia is the latest drug to hit the market in the growing list of RSV drugs targeting pregnant women and babies to protect against RSV in young children.
In the U.S., RSV season begins in October and lasts through April. The virus usually causes mild cold-like symptoms. However, in some cases, it can lead to hospitalization and death in infants and the elderly.
By age 2, 97% of all babies have been infected with RSV, which confers partial immunity and makes subsequent infections less severe.
The disease burden for infants can be serious. In the U.S., RSV infection is the leading cause of infant hospitalization among babies under age 1. A very small percentage of children — about 25 or fewer infants per year — will die from the virus, according to one Centers for Disease Control and Prevention (CDC) study.
The CDC currently recommends that pregnant women get Pfizer’s RSV vaccine Abrysvo to protect their babies in the first months of life — even though clinical trials and a post-marketing study showed that pregnant women who got the shot were more likely to experience preterm births.
The agency recommends newborns whose mothers didn’t receive the RSV shot get a monoclonal antibody shot.
Unlike a vaccine, maternal vaccination or monoclonal antibodies provide babies with “passive immunity” — antibodies to fight the virus that weren’t created by their own immune systems — by binding to and neutralizing antigens, as antibodies would.
Enflonsia differs from Beyfortus in that it binds to a different site. There is one other monoclonal antibody, palivizumab (Synagis), on the market, but it’s approved only for the prevention of RSV in infants 6 months and younger who were born prematurely or other infants with certain comorbidities.
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Merck said it anticipates orders will begin in July and it will be prepared to ship out the drug for the 2025-2026 RSV season, pending recommendation by the CDC.
The CDC vaccine advisory committee, which makes vaccine recommendations, is scheduled to meet June 25-27 to make recommendations for Enflonsia and several other shots.
However, following Monday’s announcement by U.S. Health and Human Services Secretary Robert F. Kennedy Jr. that 17 committee members will be retired, effective immediately, it’s unknown if the new committee will recommend the shot this month.
Related articles in The Defender- FDA Advisers Bullish on RSV Shots for Kids, Even as Safety Signals Shut Down Moderna Trials
- Pregnant Women Who Took Pfizer’s RSV Vaccine Were More Likely to Give Birth Prematurely
- GSK and Pfizer RSV Vaccines for Pregnant Women Increased Risk of Preterm Births — GSK Ended Its Trials, but FDA Approved Pfizer Shots
- FDA Shrugs Off Concerns About Premature Births, Approves Pfizer RSV Vaccine for Pregnant Women
