The Food and Drug Administration is refusing to consider Moderna's application for a new flu vaccine made with Nobel Prize-winning mRNA technology, the company announced Tuesday.

The Food and Drug Administration on Wednesday backtracked on its refusal to review Moderna’s mRNA flu vaccine, while making an unusual public defense of its decisions on drug approvals.

The agency told Moderna that its application didn’t contain an “adequate and well-controlled” trial because the control arm didn’t reflect the “best-available standard of care in the United States at ...