The move came after the Food and Drug Administration authorized use of the vaccine in adults 18 years and older last week, and after the agency’s Advisory Committee on Immunization Practices, or ACIP, then voted 12 to 0 in support of the shot.

Most vaccine providers wait until the CDC signs off before giving shots. The agency usually follows the counsel of the ACIP, which is composed of medical and public-health experts and advises the CDC and develops recommendations for vaccine use generally in the U.S.

Novavax’s vaccine requires two doses given three weeks apart. It will provide a new option for people who have avoided vaccination with gene-based messenger RNA vaccines made by

Moderna Inc.

and

Pfizer Inc.

and partner

BioNTech SE.

“If you have been waiting for a Covid-19 vaccine built on a different technology than those previously available, now is the time to join the millions of Americans who have been vaccinated,” said CDC Director

Rochelle Walensky.

Novavax’s protein-based vaccine uses a relatively older and more tested technology that serves as the backbone of vaccines for hepatitis B and the human papillomavirus, or HPV.

The U.S. government said last week that it had bought 3.2 million doses of Novavax’s vaccine, but it isn’t known how much demand there will be for it in the U.S.

The FDA has authorized Novavax’s shot as an initial inoculation for people who haven’t been vaccinated. About one-third of U.S. adults haven’t been fully vaccinated, and about 10% have only received one dose, according to CDC data.

“We really need to focus on that population, with the hope that perhaps this protein subunit vaccine will change them over from being unvaccinated to vaccinated,” said Oliver Brooks, an ACIP member and the chief medical officer at Watts Healthcare Corporation in Los Angeles.

A CDC-sponsored survey of 541 unvaccinated adults early this year found that 16% of people said they would probably or definitely take a protein-based vaccine like Novavax’s, compared with 52% who said they would likely not take it, an agency official said in a presentation to the ACIP.

The FDA hasn’t yet authorized it as a so-called booster shot to enhance immunity to Covid-19. Nearly half of U.S. adults haven’t yet received a third or fourth booster shot, providing a far larger market that Novavax has said it hopes to win authorization for.

Novavax has struggled with manufacturing issues that have delayed its vaccine getting on the market.

Testing found the shot was 90.4% effective at preventing Covid-19 symptoms in clinical trials. However, the studies were conducted before the Omicron variant became dominant.

Novavax has said that its vaccine loses some potency against the Omicron variant after two doses, but that a third booster shot is likely to provide strong protection.

The shot also carries an increased risk of the heart-inflammation conditions myocarditis and pericarditis, with symptoms usually appearing within 10 days after vaccination, the FDA has said. An increased risk of heart inflammation has also been linked to the Moderna and Pfizer-BioNTech vaccines.

A CDC official told the ACIP that the benefits of vaccination generally outweigh the risks of the heart-related side effects.

National health officials in Europe, Asia and elsewhere have authorized Novavax’s shot over the past several months. As of June 30, 1.07 million Novavax vaccine doses have been administered globally, Novavax said in a presentation to the ACIP.

Write to Joseph Walker at joseph.walker@wsj.com