WASHINGTON, Aug. 6 -- The FDA approved the first mRNA-based flu vaccine in United States history on Tuesday, granting Moderna's mFlusiva clearance for adults 50 and older, an advance that its manufacturer says is ready for this fall. Whether most older Americans can actually afford to get it is an entirely different question.

The Food and Drug Administration granted full approval to mFlusiva for adults between 50 and 64, and accelerated approval for adults 65 and older, a split that reflects lingering questions about how well the shot performs in the highest-risk group. In late-stage trials involving more than 40,000 older adults across 11 countries, mFlusiva demonstrated roughly 27 percent greater effectiveness against influenza than st...