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The Food and Drug Administration on Wednesday granted emergency use authorization to a booster shot for the coronavirus vaccine from Novavax.

The agency said people 18 and older can receive the booster six months after completing their primary series of any coronavirus vaccine authorized in the United States.

That means the Novavax vaccine can be used as a booster for the messenger RNA vaccines manufactured by Moderna and by Pfizer and its German partner, BioNTech. The Novavax shot was developed using a more traditional, protein-based technology and was the first such coronavirus vaccine in the United States using that approach.

The updated mRNA boosters target both the original coronavirus and the omicron BA.4 and BA.5 subvariants while the Novavax booster targets only the original strain. But the company has said its booster shot provides protection against several strains, including BA.5, which is the dominant version in the United States.

The FDA said it was authorizing the booster for adults who cannot receive one of the mRNA boosters because the shot is not "accessible or clinically appropriate."

Some people are keen to get the Novavax booster because they are allergic to an ingredient in mRNA vaccines or simply prefer the more traditional technology at the core of Novavax's shot.

Rochelle Walensky, director of the Centers for Disease Control and Prevention, is expected to recommend the booster shortly. That is the last step before the shots can be administered.

Stanley C. Erck, president and CEO of Novavax, said in a statement that "offering another vaccine choice may help increase covid-19 booster vaccination rates for these adults." He noted that fewer than half of adults eligible for boosters have received one, according to the CDC.

Novavax, a Gaithersburg, Md., biotechnology company, has been a straggler in the vaccine race. One reason is that the process it uses takes longer than the one used for mRNA vaccines. But Novavax also experienced significant delays because of manufacturing issues throughout the development of the vaccine.

Novavax's initial, two-shot regimen employs a technology used to make vaccines against influenza and shingles. The Moderna and Pfizer-BioNTech vaccines use mRNA technology to instruct the body's cells to build a protein. A vaccine from Johnson & Johnson uses a harmless cold virus.

The FDA authorized Novavax's two-dose primary series for adults aged 18 and older in July and for adolescents 12 to 17 in August.

CDC data shows demand for the Novavax vaccine has lagged, with about 35,000 doses administered. That compares with hundreds of millions of doses of the mRNA initial shots and boosters that have been given.