Reversing an earlier refusal, the FDA has approved Moderna's first mRNA flu shot for adults 50 and older. Here's what the major decision means as Health Canada considers its own approval of the vaccine.

The clearance paves the way for the first preventive flu shot based on mRNA technology and completes a dramatic regulatory turnaround.

The FDA’s approval is welcome news, and will hopefully open the door to much faster production of an effective vaccine to counter any future flu pandemic.

The vaccine, developed by Moderna, is approved for adults ages 50 to 64 and those 65 and older, who face the highest risk of severe illness.