A woman receives a Johnson & Johnson COVID-19 vaccine in a clinical study in Durban, South Africa.  (PHOTO: GETTY IMAGES)

But the FDA review also hinted that a formidable messaging challenge may lie ahead.

After the spectacular and relatively straightforward 90-plus percent effectiveness of the first two coronavirus vaccines that were authorized, the Johnson & Johnson results are more nuanced. It was more than 80 percent effective at preventing severe illness, including in areas of the world where concerning variants are circulating, but only 66 percent protective overall when moderate cases were included.

But Johnson &Johnson's vaccine can be stored at normal refrigeration temperatures for at least three months, making its distribution considerably easier than the authorized vaccines made by Moderna and Pfizer-BioNTech, which require two doses and must be stored at frigid temperatures.