RSV monoclonal Beyfortus was 90% effective at preventing hospitalizations in children this winter: CDC

A new monoclonal antibody product to protect against respiratory syncytial virus was 90% effective at preventing little children from being hospitalized with RSV, according to new data from the first season it was in use.

The data, published Thursday in Morbidity and Mortality Weekly Report, a Centers for Disease Control and Prevention publication, looked at how well Beyfortus worked in the children whose parents managed to secure a scarce dose of the drug. These are the first real-world data showing how effective the product was in the United States.

“It’s basically really good news,” said Sean O’Leary, a professor of pediatrics at the University of Colorado School of Medicine and a pediatric infectious diseases specialist at Children’s Hospital Colorado. “In some clinical trials we see some level of effectiveness … but then when it’s rolled out in the real world, for any number of potential reasons, things don’t work out as well as they had in the clinical trials.”

“In this case, it’s great. The point estimate they came up with is even better than in the clinical trials,” said O’Leary, who was not involved in the study.

AstraZeneca and Sanofi jointly developed Beyfortus, which is sold in the U.S. by Sanofi.

Pooled data from clinical trials suggested Beyfortus was 79% effective at preventing infections that were serious enough to require medical care, and 81% effective at preventing hospitalizations due to RSV in the 150 days after administration.

Authors of the new study noted the median length of follow-up in these children was 45 days. They cautioned the effectiveness of Beyfortus would likely decline over the course of an entire RSV season because the antibodies it contains degrade over time.

The analysis is based on data from the CDC’s New Vaccine Surveillance Network and looked at RSV hospitalizations during the period from the beginning of October through the end of February.

Beyfortus was approved by the Food and Drug Administration last summer and rolled out in the fall for use in any child born during RSV season or who was under 8 months of age when RSV season began. Children at high risk of becoming severely ill if they contract RSV are also eligible to get it in their second RSV season if they are between the ages of 8 months and 19 months.

RSV can infect people of any age. In healthy adults, it causes what can be a nasty cold. But in older adults and young children — especially young babies whose lungs are still developing — the infection can be alarming. Though deaths are rare, babies and little kids infected with RSV can struggle to breathe, sending families to hospitals for care. Between 50,000 and 80,000 children under the age of 5 are hospitalized every year for RSV in the U.S.

Beyfortus does not prevent all infections with RSV. Children can still get infected and may develop a cough or bronchiolitis, inflammation of the airways of the lung. “But it greatly decreases the severity and will prevent a lot of lower respiratory tract infections and prevent a lot of hospitalizations,” O’Leary said.

Monoclonal antibodies have been used to protect very high-risk children from RSV for some time, but the product, Synagis, is expensive and requires monthly administrations. Beyfortus was designed for universal use in young children and is given in a single dose. At $495 a dose, it too is expensive, though a fraction of the cost of Synagis.

The high cost of Beyfortus posed challenges for birthing hospitals and pediatric practices, which need to purchase product in advance and bill for its use after the fact. There were concerns about how best to deploy the product, and fears that given the short lead time between the FDA approval and the start of RSV season, that systems would not be in place to roll it out to all children.

O’Leary said people in the pediatric care sphere moved heaven and earth to try to get delivery systems in place last fall. “Particularly at the health systems and individual private practice levels, hospital levels, there was a ton of work that went into trying to make this happen. Because there were so many potential barriers that had to be overcome,” he said.

In the end, though, AstraZeneca and Sanofi underestimated what the demand for the product would be and shortages quickly materialized. In late October, the CDC recommended rationing the available doses, saying children under 6 months of age should be prioritized.

Sanofi said in early February that the two companies are working to ensure there is adequate supply to meet U.S. demand next RSV season. “Beyfortus is being manufactured well in advance of the RSV season, with the vast majority of doses expected to be available before October,” the company said in a statement.

The CDC now estimates that about 40.5% of eligible babies received Beyfortus this winter.

Rachel Fusfeld, who gave birth in August 2023, sought in vain to find the shot for her son.

“At a certain point our pediatrician told us that he’d never get it due to lack of supply,” she said in an email. “This was so disappointing but I knew that every day without getting sick meant he was less vulnerable to serious illness, too.”

O’Leary hopes the problems of the first year’s rollout will be ironed out in time for the next RSV season. He noted another RSV monoclonal, being developed by Merck, is working its way to the market, and suggested these products could fundamentally change the impact RSV has on children, in the way rotavirus vaccines have.

“When I trained in the late ’90s as a pediatric resident, we had flu season, we had RSV season, and we had rotavirus season. And sometimes they overlapped and it was a mess,” O’Leary said. “And after the rotavirus vaccine was rolled out, we didn’t have a rotavirus season anymore. And I think RSV could go in a very similar place.”

Correction: An earlier version of this article incorrectly listed AstraZeneca as the sole developer of Beyfortus.