A fourth Covid vaccine is cleared for use in the United States.

The Centers for Disease Control and Prevention recommended on Tuesday that a newly authorized vaccine from Novavax be used as an option for adults seeking a primary immunization against the coronavirus. Dr. Rochelle Walensky, the C.D.C.’s director, signed off on the recommendation of a panel of vaccine experts that had unanimously endorsed the vaccine on Tuesday afternoon. The decision removes the final regulatory hurdle for the fourth Covid-19 shot authorized in the United States. The Novavax vaccine is expected to play a limited role in the country’s immunization campaign, at least initially. The Food and Drug Administration authorized it last week as a primary immunization for adults, but has not yet considered it for a booster shot. Novavax, a Maryland pharmaceutical company, received significant federal funding to develop its shot but fell far behind in the vaccine race. Even so, it is hoping the shot will appeal to Americans who have so far declined to be vaccinated. Somewhere between 26 and 37 million adults in the United States have not yet received a coronavirus vaccine, according to survey data presented at the meeting by Dr. Katherine Fleming-Dutra of the C.D.C. But in the countries where it has been available for months, Novavax’s shot has yet to have a significant effect on vaccination rates. Novavax said in its presentation to the panel on Tuesday that just over a million doses of its vaccine had been administered worldwide through the end of June, a tiny number compared with the reach of competing shots. The Biden administration said last week that it would buy 3.2 million doses of Novavax’s two-shot vaccine, enough to fully vaccinate 1.6 million people in the United States. The vaccine is not yet available in pharmacies and other clinics that administer shots. Novavax said in a news release after the vote that the F.D.A. had signed off on the first doses intended for the United States and that the company expected to ship its vaccine to a distribution center “in the coming days.” With two doses spaced three weeks apart, the vaccine was found to be highly protective against infection and severe disease from the coronavirus in clinical trials. But those were conducted before the emergence of the Omicron variant, which has sharply reduced the effectiveness of other authorized vaccines in preventing infections. Novavax’s vaccine works differently from the three Covid vaccines previously authorized in the United States. It provokes an immune response with nanoparticles made up of proteins from the surface of the coronavirus that causes Covid-19. Similar protein-based vaccines have been used globally for decades. Dr. Evelyn Twentyman of the C.D.C. presented results from a survey suggesting that relatively few unvaccinated Americans would choose Novavax. Just 16 percent of unvaccinated respondents said they “probably” or “definitely” would get a protein-based Covid vaccine, and 50 percent of unvaccinated respondents said they “probably” or “definitely” would not. At the meeting on Tuesday, C.D.C. officials reviewed clinical trial data that has linked the vaccine to an elevated but small risk of developing forms of heart inflammation known as myocarditis and pericarditis. Dr. Filip Dubovsky, the chief medical officer at Novavax, said at the meeting that the company had been monitoring cases of heart inflammation in countries where the vaccine was already authorized. Surveying just over a million doses, the company has identified 17 definite or probable cases of myocarditis and pericarditis, he said.

A fourth Covid vaccine is cleared for use in the United States.

The Centers for Disease Control and Prevention recommended on Tuesday that a newly authorized vaccine from Novavax be used as an option for adults seeking a primary immunization against the coronavirus.

Dr. Rochelle Walensky, the C.D.C.’s director, signed off on the recommendation of a panel of vaccine experts that had unanimously endorsed the vaccine on Tuesday afternoon. The decision removes the final regulatory hurdle for the fourth Covid-19 shot authorized in the United States.

The Novavax vaccine is expected to play a limited role in the country’s immunization campaign, at least initially. The Food and Drug Administration authorized it last week as a primary immunization for adults, but has not yet considered it for a booster shot.

Novavax, a Maryland pharmaceutical company, received significant federal funding to develop its shot but fell far behind in the vaccine race. Even so, it is hoping the shot will appeal to Americans who have so far declined to be vaccinated.

Somewhere between 26 and 37 million adults in the United States have not yet received a coronavirus vaccine, according to survey data presented at the meeting by Dr. Katherine Fleming-Dutra of the C.D.C.

But in the countries where it has been available for months, Novavax’s shot has yet to have a significant effect on vaccination rates. Novavax said in its presentation to the panel on Tuesday that just over a million doses of its vaccine had been administered worldwide through the end of June, a tiny number compared with the reach of competing shots.

The Biden administration said last week that it would buy 3.2 million doses of Novavax’s two-shot vaccine, enough to fully vaccinate 1.6 million people in the United States. The vaccine is not yet available in pharmacies and other clinics that administer shots. Novavax said in a news release after the vote that the F.D.A. had signed off on the first doses intended for the United States and that the company expected to ship its vaccine to a distribution center “in the coming days.”

With two doses spaced three weeks apart, the vaccine was found to be highly protective against infection and severe disease from the coronavirus in clinical trials. But those were conducted before the emergence of the Omicron variant, which has sharply reduced the effectiveness of other authorized vaccines in preventing infections.

Novavax’s vaccine works differently from the three Covid vaccines previously authorized in the United States. It provokes an immune response with nanoparticles made up of proteins from the surface of the coronavirus that causes Covid-19. Similar protein-based vaccines have been used globally for decades.

Dr. Evelyn Twentyman of the C.D.C. presented results from a survey suggesting that relatively few unvaccinated Americans would choose Novavax. Just 16 percent of unvaccinated respondents said they “probably” or “definitely” would get a protein-based Covid vaccine, and 50 percent of unvaccinated respondents said they “probably” or “definitely” would not.

At the meeting on Tuesday, C.D.C. officials reviewed clinical trial data that has linked the vaccine to an elevated but small risk of developing forms of heart inflammation known as myocarditis and pericarditis.

Dr. Filip Dubovsky, the chief medical officer at Novavax, said at the meeting that the company had been monitoring cases of heart inflammation in countries where the vaccine was already authorized. Surveying just over a million doses, the company has identified 17 definite or probable cases of myocarditis and pericarditis, he said.