WASHINGTON, United States -- The US Food and Drug Administration on Wednesday authorized Novavax's Covid-19 shot for use in people aged 18 and over, expanding Americans' options for vaccination against the disease.

The vaccine -- which is produced by an American company -- is already available in dozens of other countries. Just three Covid shots, two of them based on messenger RNA technology, have been previously authorized in the United States.

The emergency authorization of the Novavax vaccine "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," the FDA's commissioner Robert Califf said in a statement.

Administered in two doses three weeks apart, the Novavax vaccine uses a more conventional technology than the mRNA shots, which have been the subject of numerous conspiracy theories, though they have been proven to be safe.

Novavax's vaccine contains part of the virus that triggers an immune response -- the approach used by many other common shots.

In clinical trials involving thousands of people in the United States and Mexico, it was 90 percent effective against symptomatic cases of Covid-19. However, the trials were conducted prior to the emergence of the Delta and Omicron variants of the disease.

The vaccination rate of adults in the United States has stagnated at around 77 percent.

The US government had announced this week that it had purchased 3.2 million Novavax doses in anticipation of the FDA decision.