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A panel of independent vaccine experts recommended Tuesday that the Food and Drug Administration authorize a vaccine developed by Novavax, paving the way for the fourth coronavirus shot in the United States.

The timing of when doses of the Novavax vaccine will be available was not clear. A decision by the agency is unlikely to happen immediately because a review of data regarding manufacturing is ongoing. Novavax submitted updated manufacturing information to support its vaccine authorization Friday.

The Novavax shot is a protein-based vaccine based on a traditional technology used against influenza and shingles. Many experts are eager to add another vaccine to the toolbox, particularly because the Johnson & Johnson vaccine is now recommended only for people who cannot or will not take messenger RNA vaccines because of the rare risk of a potentially life-threatening side effect.

“It’s important to have choices in vaccine platforms in a pandemic that is constantly evolving,” said Gregory Poland, director of the Mayo Clinic’s Vaccine Research Group and a paid consultant to Novavax.

But Novavax’s two-shot regimen is likely to have a limited impact as it arrives more than a year after other highly effective vaccines became available. Most of the population eligible for the shot — people 18 and older — are already vaccinated. About 27 million adults in the United States have not received any shot, according to data from the Centers for Disease Control and Prevention.

Experts on the committee debated if the Novavax vaccine should even receive emergency use authorization, given other highly effective and safe options and some expressed skepticism about whether vaccine hesitant people would embrace it.

“Having a protein-based alternative may be more comfortable for some in terms of their acceptance of vaccine,” said Peter Marks, the FDA’s director of the Center for Biologics Evaluation and Research. “I will use this as a moment on the bully pulpit to say: We do have a problem with vaccine uptake that is very serious in the U.S., and anything we can do to make people more comfortable to accept these potentially lifesaving products is something we are compelled to do.”

The first vaccines to be authorized in the United States were based on messenger RNA technology, which instructs the body’s cells to build proteins that teach the immune system to halt the coronavirus. The technology had been under development for years, but the pandemic was the first time it was authorized — and the shots have now been widely used.

A niche group of people who can’t or won’t take the messenger RNA vaccines may embrace the Novavax shots, but the ultimate use of the vaccine is probably as a booster. Going forward, the company plans to seek authorization as a booster and for use in adolescents and children.

The Novavax vaccine was shown to be 90% effective in a clinical trial run before the emergence of the omicron variant of the coronavirus. The results were announced more than a year ago, but the vaccine has repeatedly been delayed by manufacturing challenges.

Several experts noted that the data look similar to those that supported the mRNA vaccines when they were authorized over a year ago.

“It is disappointing that we don’t have more updated information, because we’re looking at the efficacy against strains that don’t exist anymore,” said Eric J. Rubin, an infectious disease specialist at Brigham and Women’s Hospital in Boston. Even so, he said the case for the vaccine was “pretty compelling” considering some people who have avoided mRNA vaccines have shown interest in Novavax.

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