(Reuters) -British drugmaker GSK said on Monday it had submitted an application to the U.S. Food and Drug Administration to extend the use of its respiratory syncytial virus (RSV) vaccine to adults under the age of 50 at a higher risk of the disease.

If approved in the world's biggest pharmaceuticals market, GSK's vaccine, Arexvy, would be competing for a market share in the 18-49 age group with Moderna's mRESVIA and Pfizer's Abrysvo.

RSV is a common respiratory virus that causes seasonal infections such as the flu and is a leading cause of pneumonia and death in infants and older adults.

GSK's vaccine is currently approved in the U.S. for preventing RSV-related disease in adults aged 60 and above, and in at-risk adults aged 50 to 59. While it had a lead in approvals initially, it has since lagged behind rivals and sales have fallen sharply.

The FDA is expected to make a decision on the expanded use of Arexvy in the first half of 2026, GSK said. European regulators are also expected to rule on a similar application by then.