The Food and Drug Administration on Friday expanded the approval of GSK‘s respiratory syncytial virus vaccine to adults ages 50 to 59 who are at increased risk of getting severely sick from the potentially lethal virus. 

The shot, called Arexvy, is the first vaccine cleared by the FDA to protect that population from RSV. The agency first approved GSK’s jab in May 2023 for patients 60 and above, who are more vulnerable to severe cases of the virus. 

RSV causes thousands of hospitalizations and deaths among seniors each year, according to data from the Centers for Disease Control and Prevention. But the virus can also cause severe illness in adults 50 and up — or even younger — with underlying chronic conditions such as asthma, diabetes and congestive heart failure.

About 13 million Americans ages 50 to 59 are at high risk of severe illness from RSV, said Phil Dormitzer, GSK’s head of vaccines research and development and infectious disease research, in an interview. 

“It’s useful both because, of course, you can meet the medical needs of that age group,” Dormitzer told CNBC, “but it’s also nice for pharmacists to have a single vaccine that they can administer to a wider population, so that provides simplicity.”

GSK’s shot won’t reach that new patient population just yet. An advisory panel to the CDC will vote later in June on recommendations for GSK’s vaccine, along with a rival shot from Pfizer and a newly approved jab from Moderna. 

The FDA’s expanded approval could help GSK maintain its dominance in the RSV market later this fall and winter, when the virus typically spreads more widely in the U.S. The British drugmaker’s shot booked around £1.2 billion in sales last year, outpacing the $890 million (about £699 million) in revenue that Pfizer’s vaccine raked in. 

GSK Chief Commercial Officer Luke Miels said on an earnings call in May that the company remains “very confident” that Arexvy can bring in more than £3 billion in peak annual sales over time.

Dormitzer said GSK had a successful last RSV season, but noted that the company will always “take the competition seriously.” 

He said Arexvy showed strong efficacy in patients who have underlying medical conditions. 

In a late-stage trial, a single dose of the shot elicited an immune response in high-risk adults ages 50 to 59 which wasn’t worse than that observed in people 60 and above. 

A previous late-stage trial on that older age group found the shot was nearly 83% effective at preventing lower respiratory tract disease caused by RSV and around 94% effective at preventing severe disease. 

Safety data in adults ages 50 to 59 was also consistent with data in adults 60 and above, according to GSK. Side effects included fatigue, headache and muscle pain, among others, which were mostly mild to moderate in severity. 

A single dose of GSK’s shot was only slightly less effective in adults 60 and up after two seasons of the virus, showing 67.2% efficacy against lower respiratory tract illness. Dormitzer said the company will test the vaccine’s efficacy over three RSV seasons to see if it can provide even longer protection. 

GSK is also studying Arexvy in other patient groups to expand the shot’s reach in the future. The company is expected to announce trial data later in 2024 on two separate patient groups: people ages 18 to 59 who are at increased risk of severe RSV, and adults with weakened immune systems.

Dormitzer added that the company is also expanding the shot’s reach in other countries. Regulatory agencies in Europe, Japan and other areas are currently reviewing GSK’s application to expand Arexvy’s approval to high-risk adults ages 50 to 59. 

GSK’s shot is approved in nearly 50 countries, a spokesperson for the company told CNBC.