Oct 8 (Reuters) - GSK said on Tuesday that its

Arexvy respiratory syncytial virus vaccine was 43.3% effective

in preventing severe illness in its third season after patients

received the shot.

That compares with 94.1% effectiveness in preventing severe

RSV in the first season and 64.2% a year later, according to

data from GSK's Phase 3 clinical trial. The trial initially

enrolled about 25,000 volunteers in 17 countries.

Arexvy is one of three RSV vaccines approved for use,

alongside shots made by Pfizer ( PFE ) and Moderna ( MRNA ),

with GSK's shot so far dominating the market.

In the U.S., the vaccines are currently recommended as a

one-time shot for adults aged 75 and older, and for those ages

60 to 74 who are at higher risk of severe illness due to health

conditions or other factors.

"We are excited by these new data which show that a single

dose of Arexvy could help protect millions of older adults at

risk of RSV disease over three seasons to benefit public

health," Tony Wood, GSK's chief scientific officer said in a

statement.

GSK said it was continuing to supply data to regulators to

help them determine if or how often patients should be

re-vaccinated.

RSV typically causes cold-like symptoms but is also a

leading cause of pneumonia in toddlers and older adults, leading

to 177,000 hospitalizations and 14,000 deaths in the United

States annually.