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 authorised in the US.

The emergency authorisation of the Novavax vaccine “offers adults in the US 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 US and Mexico, it was 90% 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%.

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