The Food and Drug Administration on Wednesday issued an emergency use authorization for Novavax's coronavirus vaccine, making it the fourth COVID vaccine available for people over 18 years old.

The big picture: The Biden administration announced this week that it had purchased over 3 million doses of Novavax's vaccine in anticipation of the FDA authorizing the vaccine.

Details: The Novavax protein-based vaccine — which is easier to store than Moderna or Pfizer's vaccine — is administered in two doses taken three weeks apart.

• The vaccine uses a protein from the virus along with an adjuvant, which "enhance the immune response of the vaccinated individual," the FDA says.

Flashback: In June, the FDA said that Novavax's vaccine was shown to be 90% effective at preventing new coronavirus cases, but carried the possible risk of causing heart inflammation, particularly in young men, Axios' Adriel Bettelheim reported.

What they're saying: "Authorizing an additional COVID-19 vaccine expands the available vaccine options for the prevention of COVID-19, including the most severe outcomes that can occur such as hospitalization and death,” said FDA commissioner Robert Califf.

• "Today’s authorization offers adults in the United States who have not yet received a COVID-19 vaccine another option that meets the FDA’s rigorous standards for safety, effectiveness and manufacturing quality needed to support emergency use authorization."