(The Hill) — The Food and Drug Administration (FDA) on Wednesday authorized Novavax’s COVID-19 shot for emergency use, adding a fourth vaccine to the U.S. arsenal.

The agency cleared the vaccine for adults ages 18 and older. The shot will be administered as a two-dose primary series, three weeks apart.

Novavax’s vaccine could appeal to people who have been reluctant to receive one of the mRNA vaccines, as the protein-based shot was manufactured using a fundamentally different process. But about 77 percent of adults have already received two doses of an mRNA shot, so the potential market is small.

The U.S. is not short on doses from any of the Pfizer-BioNTech or Moderna vaccines that have been available for more than a year, and those manufacturers are already working with administration officials to formulate a variant-specific booster shot for the fall.

Still, the vaccine comes as the White House is urging people to get vaccinated and boosted to protect against a wave of severe infection from the BA.4 and BA.5 subvariants of omicron. 

Novavax’s shot “provides another alternative for adults and adds another vaccine to the COVID-19 vaccine supply for the United States. The American public can trust that this vaccine, like all vaccines that are used in the United States, has undergone the FDA’s rigorous and comprehensive scientific and regulatory review,” said Peter Marks, director of the FDA’s vaccine division.

In clinical trials, the vaccine was about 90 percent effective at preventing mild, moderate and severe COVID-19. But, critically, the trials were conducted before the omicron variant became dominant.

The company has since submitted updated data to the FDA, and publicly said the vaccine shows “broad” response to the currently circulating variants.

Novavax relies on more traditional vaccine technology, similar to those used to protect against flu and for routine childhood immunizations against whooping cough and meningitis. The two-dose vaccine can be stored at normal refrigerator temperatures, making it easier to ship and use. 

The Maryland-based Novavax secured a $1.8 billion contract from the former Trump administration’s Operation Warp Speed early in the pandemic to scale up research and purchase up to 100 million doses.

The Biden administration earlier this week announced it was purchasing 3.2 million doses.

Novavax originally wanted to ask the FDA for authorization by May 2021, but was beset by manufacturing problems and struggled to scale up production. It wasn’t able to file until the end of January 2022. 

The vaccine is currently being manufactured by the Serum Institute of India, the world’s largest vaccine producer.

An FDA advisory panel voted in favor of recommending authorization last month, but the process dragged out because the agency was conducting a review of the company’s manufacturing process.

The FDA this week said Novavax is expected to finish its quality testing “in the next few weeks.”

Once the Centers for Disease Control and Prevention signs off, vaccination can begin. The agency’s Advisory Committee on Immunization Practices is scheduled to meet on July 19.