Sanofi and British-Swedish firm AstraZeneca, which jointly developed the drug nirsevimab, marketed as Beyfortus, say it is the first treatment to prevent severe illness from RSV in infants.
The virus infects around nine out of 10 children by the age of two and is the most common cause of bronchiolitis, a respiratory infection that is normally mild but in some cases can make infants seriously ill.
The results showed an 83 percent reduction in hospitalizations for infants with RSV-related illnesses who received a single dose of nirsevimab, compared to a control group who did not receive the treatment.
The results suggest that "the overall burden on healthcare systems could be reduced significantly if all infants receive nirsevimab," Sanofi said.
Global medical costs worldwide were estimated to be 4.8 billion euros ($5.2 billion) in 2017, it added.
While not a vaccine, nirsevimab is a monoclonal antibody treatment that has a similar aim: to give protection against RSV with a single injection.
Peter Openshaw, a specialist in lung immunology at Imperial College London, pointed out that the trial data was collected when RSV was rebounding after a dip in cases during Covid lockdowns.
The data "adds to the evidence that use of long-acting monoclonal antibody may prevent moderate to severe RSV disease after a convenient single dose," he said, adding that further studies were needed for older children.
"The cost of nirsevimab will be a critical determinant of how widespread its use can be," Openshaw said.
The drug has already been approved in the European Union, the U.K. and Canada, and an application in the United States is under review.
Several major pharmaceutical firms have developed vaccines for RSV in adults, and are currently racing to get them approved.
Last week, the U.S. became the first country to approve an RSV vaccine, giving the greenlight to GSK's Arexvy for adults aged 60 or older.
More attention has been paid to RSV since a combination of the virus, COVID-19 and influenza dubbed a "tripledemic" put pressure on hospitals in several countries during the Northern Hemisphere's last winter.
