Adds context in paragraphs 1-4, expert comment in paragraph 7

By Christy Santhosh and Mariam Sunny

June 18 (Reuters) - Advisers to the U.S. FDA are meeting on Thursday to evaluate Moderna's MRNA.O flu vaccine for adults aged 50 and older, viewed as a test of whether the agency is reverting to its longstanding approach to vaccine reviews after recent leadership turnover.

Under former FDA Commissioner Marty Makary and his lieutenants, the FDA rejected Moderna's initial application for the shot before reversing course.

The Vaccines and Related Biological Products Advisory Committee, which began discussions earlier on Thursday, is set to vote on whether the benefits of the mRNA-based shot, called mFlusiva, outweigh its risks in the intended population.

The advisers are reviewing data submitted by Moderna, which it says shows its vaccine outperformed standard-dose shots in adults aged 50 to 64. It argues a separate, smaller study shows the vaccine also generated a strong immune response compared with a high-dose flu vaccine in adults 65 and older.

​In briefing documents published ahead of the meeting, FDA staff reviewers said data demonstrating an immune response to Moderna's flu shot might support effectiveness in adults aged 65 and older.

The documents, however, flagged concerns over the shot's effectiveness in immunocompromised people and very frail older adults who were excluded from trials.

The mRNA vaccine technology allows for more rapid manufacturing, which carries the potential to facilitate a more precise seasonal influenza strain selection, said Joseph Kulinski, chair of the FDA's Review Committee.

Traditional vaccines require several months from strain selection to available supply. Because of that, the flu strain selected each year is chosen months earlier than the COVID vaccine strain for the same season.

The Massachusetts-based vaccine maker is under pressure to prove the long-term commercial potential of its mRNA technology, which powered its blockbuster COVID-19 vaccine and helped establish the platform during the pandemic.

The company is seeking a traditional approval for the shot for adults 50 to 64 ⁠years old, ​and an accelerated approval, which requires the company to conduct additional studies to confirm the vaccine's benefit, for older adults.

Moderna has agreed to run an additional study and submit more data in adults 65 and older if it gets approval for that age group.

REGULATORY TURMOIL

Under Health Secretary Robert F. Kennedy Jr., a longtime anti-vaccine activist, and Makary, the agency stepped up its scrutiny of vaccines, narrowing approvals, rather than allowing the CDC and its advisers to make those recommendations.

Makary and then-top vaccine official Vinay Prasad left the agency following a series of controversies over vaccine and rare-disease drug reviews, including Moderna's flu vaccine application.

Acting Commissioner Kyle Diamantas has since moved to stabilize agency operations and repair relations with the biotech sector after months of upheaval.

If approved, Moderna's shot would be the first seasonal flu shot in the U.S. made with mRNA technology. It would compete with flu vaccines from Sanofi SASY.PA, GSK GSK.L, CSL Seqirus CSL.AX and AstraZeneca AZN.L.

The FDA decision on the vaccine is expected by August 5. Jefferies analyst Andrew Tsai does not expect flu revenue for Moderna until 2027 and sees $750 million of U.S. flu and COVID-flu combo vaccine sales by 2030.