A third monoclonal antibody product to protect babies against respiratory syncytial virus was approved by the Food and Drug Administration on Monday, a development public health experts hope may eventually help to bring down the price of this effective but costly way of reducing the crushing burden of RSV illness on families and the health care system.

Merck’s clesrovimab, which will be marketed under the name Enflonsia, was approved for use in children under the age of 12 months. The wholesale price for Enflonsia has been set at $556 a dose, Merck said, which is the same price charged for its major competitor, Sanofi and AstraZeneca’s Beyfortus (nirsevimab).

While the price points are the same, the two products have differences. While there are two different-sized doses for Beyfortus, depending on a baby’s weight, there is a single dosage of Enflonsia used for all babies. That could give the product an advantage in that pediatricians’ offices wouldn’t have to worry about giving the wrong dose to a baby.

“We do think that the fact … that we have a single dose for all babies regardless of weight will just help some of those logistical issues that nirsevimab was faced with during its rollout,” Paula Annunziato, Merck’s senior vice president for clinical research told STAT in an interview. “We start off, we think, with a simpler logistical situation which we believe will make a big difference for providers and families who are seeking Enflonsia this season.”

But where Beyfortus was approved for use both in all babies during their first RSV season and high-risk babies in their second RSV season, at present Enflonsia is approved only for children in the first year they face the virus. 

Annunziato said the company is still conducting a study comparing Enflonsia to Synagis, the first RSV monoclonal brought to market. The product, made by Sobi, is only approved for infants considered at highest risk of severe illness if they contract RSV — preterm infants and babies with some health conditions. If the results are favorable, Merck will seek an extension of Enflonsia’s license to include high-risk children in their second RSV season, she said.

The Advisory Committee on Immunization Practices, the expert panel that advises the Centers for Disease Control and Prevention on how to use vaccines, was expected to vote at its next meeting in late June on whether to recommend use of Enflonsia. But Monday’s news that the entire committee had been fired and would be replaced by as yet unnamed individuals raises questions about how well the June meeting will function. According to the Health and Human Services Department, the June meeting will still take place.

Annunziato said Merck could begin taking orders for the product in July and could ship doses in time for the 2025 RSV season. “We’re really excited because we think that Enflonsia is going to be a really important prevention for families, for infants, and of course for the health care system,” Annunziato said.

The monoclonal antibody injection was shown to reduce the rate of medically attended lower respiratory infection caused by RSV by over 60% and cut the incidence of RSV hospitalization by over 84%. “Those are really meaningful differences when you consider how terrible it is to have a baby so sick they’re in a hospital,” she said.

After decades without any preventive tools for RSV, a number have reached the market in the past couple of years. Pfizer developed and licensed a vaccine for pregnant people, Abrysvo, which induces maternal antibodies that are transferred in utero to a developing fetus. It is offered to pregnant people who will give birth in the period from September through January, when their babies will be at risk of encountering RSV.

Babies born to mothers who were not vaccinated during pregnancy can get a monoclonal antibody, either at or near birth for babies born during the fall and winter, or before the start of RSV season, for babies born at other points in the year.