Pfizer said on Tuesday its respiratory syncytial virus (RSV) vaccine Abrysvo was well tolerated and generated an immune response in higher risk adults under the age of 60 similar to that in older adults, for whom the shot is already approved.

Pfizer said it plans to submit its findings from the trial to seek expanded approval of the vaccine in adults ages 18 to 59, but did not give a time frame for when it expects the data to be considered by regulators.

"This represents a real opportunity for an expansion to an age population in a risk population that wouldn't normally see the availability of this vaccine so soon," Dr. Iona Munjal, executive director of clinical vaccine R&D at Pfizer, said in an interview.

The U.S. drugmaker last year launched Abrysvo for older adults and for pregnant women to protect their babies from the virus. GSK also launched its rival vaccine Arexvy in 2023 and has dominated the RSV vaccine market over the first season they were available.

The British drugmaker is also looking to expand the age range for its shot and has already submitted data for adults ages 50 to 59 to the U.S. Food and Drug Administration. The FDA is expected to decide on whether to approve the label expansion by June 7, which could allow GSK to offer its shot in the younger age group later this year.

Pfizer did not say whether it expects to be able to expand Abrysvo's label in time for the 2024-25 respiratory virus season.

RSV, which typically causes cold-like symptoms, is a leading cause of pneumonia in toddlers and older adults.

The late-stage trial of Pfizer's shot involved 681 adults aged 18 to 59 with conditions including asthma, diabetes, and chronic obstructive pulmonary disease that increase the risk from RSV. In this group, the immune response was non-inferior to the one observed in older adults, Pfizer said.

The company said those with a higher risk profile represent 9.5% of U.S. adults 18 to 49 years of age and nearly a quarter of those between the ages of 50 and 64.

Dr. David Boulware, an infections disease specialist at the University of Minnesota, said he was not sure most Americans who fit those high risk categories would need the shot.

People who have had organ transplants, are undergoing chemotherapy for cancer, or are survivors of congenital disease would clinically benefit from the shot, he suggested.

"The clinical benefit is likely going to be relatively low for most people," he said.