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June 16 (Reuters) - Staff of the U.S. Food and Drug Administration said on Tuesday that data demonstrating a strong immune response to Moderna's flu shot may support effectiveness in adults 65 years and older.

They said Moderna's shot mFlusiva also showed superior relative vaccine efficacy versus a standard-dose flu vaccine in adults 50 to 64 years old, according to briefing documents published by the regulator ahead of an advisory meeting on Thursday.

The FDA's Vaccine and Related Biological Products Advisory Committee will convene on June 18 to vote on whether the benefits of mFlusiva outweigh the risks in adults 50 to 64 years old, and in adults 65 years and older.

Moderna is seeking a traditional approval of mFlusiva for adults 50 to 64 years old, and an accelerated approval for adults 65 years and older.

In February, the FDA had refused to initiate a review of the approval application on Moderna's choice to compare mFlusiva to an already licensed standard-dose seasonal influenza rather than the enhanced shots preferred for older adults.

After meeting with the company, the agency reversed course and accepted an amended application with the promise that Moderna would conduct an additional study in older adults once the vaccine is approved.

In the briefing documents, FDA staff flagged efficacy concerns on the standard-dose shot comparison, limited one-season data, higher short-term side effects and the need for postmarket studies in adults 65 and older.

Moderna did not immediately respond to a Reuters request for comment.