U.S. adults who haven't gotten any COVID-19 shots yet should consider a new option from Novavax — a more traditional kind of vaccine, health officials said Tuesday.

Regulators authorized the nation's first so-called protein vaccine against COVID-19 last week, but the final hurdle was a recommendation from the Centers for Disease Control and Prevention.

"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," Dr. Rochelle Walensky, CDC's director, said in a statement, endorsing an earlier decision from an influential advisory panel.

Most Americans have gotten at least their primary COVID-19 vaccinations by now, but CDC officials said between 26 million and 37 million adults haven't had a single dose — the population that Novavax, for now, will be targeting.

"We really need to focus on that population," said CDC adviser Dr. Oliver Brooks, past president of the National Medical Association. Hopefully, the vaccine "will change them over from being unvaccinated to vaccinated."

While it's unclear how many will be persuaded by a more conventional option, "I'm really positive about this vaccine," agreed fellow adviser Dr. Pablo Sanchez of Ohio State University.

The Novavax difference

All of the vaccines used in the U.S. train the body to fight the coronavirus by recognizing its outer coating, the spike protein — and the first three options essentially turn people's cells into a temporary vaccine factory. The Pfizer and Moderna vaccines deliver genetic instructions for the body to make copies of the spike protein. The lesser-used Johnson & Johnson option uses a cold virus to deliver those instructions.

In contrast, the Novavax vaccine injects copies of the spike protein that are grown in a lab and packaged into nanoparticles that to the immune system resemble a virus. Another difference: An ingredient called an adjuvant, that's made from the bark of a South American tree, is added to help rev up that immune response.

Protein vaccines have been used for years to prevent other diseases including hepatitis B and shingles.

How well it works

Large studies in the U.S., Mexico and Britain found two doses of the Novavax vaccine were safe and about 90% effective at preventing symptomatic COVID-19. When the delta variant emerged last summer, Novavax reported a booster dose revved up virus-fighting antibodies that could tackle that mutant.

Typical vaccine reactions were mild, including arm pain and fatigue, but regulators did warn about the possibility of a rare risk, heart inflammation, that also has been seen with the Pfizer and Moderna vaccines, mostly in teen boys or young men.