Adds details from press briefing paragraphs 1-5

By Mariam Sunny and Michael Erman

Feb 11 (Reuters) - A U.S. Food and Drug Administration official on Wednesday defended the agency's decision to not review Moderna's MRNA.O experimental flu vaccine application, saying the company should have given a higher strength flu vaccine to older patients in the control arm of its trial.

The official said at a press briefing that the trial - which included patients 50 and older - should have ensured that patients over age 65 receive the stronger flu vaccine recommended for that age group.

He said the agency was not making any determination on the vaccine itself and could consider the application for a younger age group were the company to resubmit.

On Tuesday evening, Moderna disclosed that the FDA had sent it a refusal-to-file letter in which it cited Moderna's choice to compare the vaccine to the standard dose flu vaccine in the trials as a reason for rejecting its application, driving shares down after hours.

Shares were off about 4% in midafternoon after declining as much as 12% earlier in the day.