(Reuters) -Moderna's shares fell 2.5% before the bell on Friday, on concerns whether a new CDC advisory panel would back the use of the company's respiratory syncytial virus vaccine in a broader age group.

The RSV shot, mRESVIA, secured U.S. approval on Thursday for expanded use in at-risk adults aged 18 to 59 years, but needs the U.S. Centers for Disease Control and Prevention's recommendation before it is made available for the age group. It is already approved for use in adults aged 60 or older.

The biotech's stock extended its decline into the second session, following the appointment of the new panel late Wednesday. Some of the new appointees have openly expressed anti-vaccine views, including against the mRNA vaccine technology on which Moderna's shots are based.

Earlier this week, Health Secretary Robert F. Kennedy Jr. fired all 17 members of the panel, known as the Advisory Committee on Immunization Practices (ACIP), saying it would "re-establish public confidence in vaccine science" - a move health experts warn could undermine public confidence in available vaccines.

The ACIP advises the CDC on who should get the shots after they are approved by the U.S. Food and Drug Administration.

The panel meeting for Moderna's RSV shot, scheduled for later this month, will be the "first time for investors to see the refreshed ACIP in action and will be important for sentiment in the vaccine sector, which continues to decline in our view," said William Blair analyst Myles Minter.

U.S.-listed shares of GSK and Pfizer fell marginally and those of smaller Novavax were down 1.7% at $7.03 in premarket trade.

The CDC currently recommends Moderna's vaccine for adults aged 75 and older, as well as for at-risk adults aged 60 to 74.

In April, the ACIP recommended the use of approved RSV vaccines in at-risk adults aged 50 to 59. However, the CDC is yet to sign off on the recommendation.

(Reporting by Mariam Sunny in Bengaluru)