Johnson & Johnson's adenovirus vector vaccine requires just one dose to effectively teach your immune system how to fight off moderate to severe disease caused by COVID-19.

U.S. regulators decided that Johnson & Johnson's single-dose vaccine provides enough protection against COVID-19 for nationwide use, with the FDA approving it for emergency use.

Trials found the shot to be 66% effective at preventing moderate to severe disease, 85% effective at preventing the most severe disease, and 100% effective at preventing hospitalizations and deaths as of 28 days post-vaccination.

This will be the third vaccine to hit the market in the U.S. Both Pfizer-BioNTech and Moderna's vaccines require two doses of an mRNA vaccine to reach their unprecedented high efficacy rates of about 95%.

Johnson & Johnson and AstraZeneca are studying a different kind of vaccine, called an adenovirus vector vaccine. It differs from the mRNA vaccines in a two key ways: the way it gets mRNA into your cells, and the way doses are stored and transported.