June 26 (Reuters) - A committee of outside experts to

the U.S. CDC are set to vote on whether to expand the age group

of adults recommended to receive respiratory syncytial virus

(RSV) vaccines this year, as GSK battles rival Pfizer ( PFE )

and new entrant Moderna ( MRNA ) to keep its top

position in the market.

Rituparna Das, Moderna's ( MRNA ) therapeutic head of respiratory

vaccines, said the company's mRESVIA shot had 50% efficacy in

preventing RSV after 18 months, speaking at the meeting of the

Centers for Disease Control and Prevention's Advisory Committee

on Immunization Practices on Wednesday. It is a lower number

than GSK and Pfizer ( PFE ) previously showed in their own clinical

trials.

Moderna ( MRNA ) shares fell around 6% in early trading.

The committee will meet Wednesday through Friday to consider

and recommend who should get RSV, influenza, COVID-19 and other

vaccines and how strong to make those recommendations.

Although the CDC generally follows the committee's votes,

the final decision rests with the agency's director.

Last year, GSK's Arexvy and Pfizer's ( PFE ) Abrysvo went

head-to-head for adults over the age of 60. GSK won two-thirds

of the market, mostly due to its contracts with retail

pharmacies, including CVS. Moderna ( MRNA ) received approval for

its shot, mRESVIA, for the same age group last month.

Headed into this autumn, GSK has FDA approval to expand use

of its vaccine to adults ages 50 to 59 at increased risk from

the illness, making it the first shot endorsed for that age

group.

Pfizer ( PFE ) said it submitted data for its shot in adults aged 18

to 59 and hopes to get the expanded approval as early as the

fourth quarter.

"Getting to that 50- to 59-year-old population early is

pretty meaningful," said Guggenheim Partners analyst Evan Wang.

"Not only does it add 10 to 15 million more high-risk

individuals to the total pool, it also is significant because

... this could give them an edge as they kind of negotiate with

retail pharmacies for contracts," Wang added.

Unlike GSK, Pfizer ( PFE ) has started the contracting season with

an FDA approval in hand for pregnant women to protect their

babies. Last year, it did not receive the maternal approval

until late August, after the vaccination campaign had begun.

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.

Combined revenue for the Pfizer ( PFE ) and GSK shots in their first

year of availability topped $2.4 billion.

Analysts, on average, expect sales of $1.59 billion for

GSK's RSV shot, $1.47 billion for Pfizer's ( PFE ) and $370 million this

year for Moderna ( MRNA ).

Pfizer ( PFE ) Chief Executive Albert Bourla has acknowledged that

Abrysvo had a "bad launch" last year, and company executives

have said they are focused on improving its performance.

Pfizer ( PFE ) recently secured a contract to supply close to 5

million doses of its vaccine in the UK over the next two years.

Britain-based GSK said it was disappointed not to have been

selected.

Protection from the Pfizer ( PFE ) and GSK shots has been shown in

trials to last at least two RSV seasons. In their clinical

trials, GSK's RSV vaccine Arexvy was 78% effective in preventing

severe RSV over a second year and Pfizer's ( PFE ) was 78% effective

through a second RSV season.

Moderna ( MRNA ) has previously cautioned against comparing its

vaccine's efficacy to rivals, noting that the trials were not

head-to-head and used different case definitions for RSV

disease.

Analysts believe the market will grow from 2023, in part

because the companies have been marketing the shots directly to

consumers since last year.

"They definitely got the low hanging fruit last year," said

Morningstar analyst Damien Conover.

Guggenheim's Wang said he expects the market to be between

$2.5 billion and $3 billion this year.