What we know about getting a COVID-19 booster shot

Nurse Solennys Fernandez vaccinated Elpiro Antonio Munoz at a clinic run by the Greater Lawrence Family Health Center.Suzanne Kreiter/Globe Staff

Days after federal regulators said most fully vaccinated Americans don’t need a COVID-19 booster shot yet, US health officials announced Wednesday that the government’s stance has changed. Now, a third round of injections of the Pfizer and Moderna vaccines could begin the week of Sept. 20 in an effort to fight waning immunity and the highly contagious Delta variant.

The reversal came two days after Pfizer and its German partner, BioNTech, filed early-stage data with the Food and Drug Administration that the drug firms said showed the safety and effectiveness of a third dose of their vaccine.

The timeline for boosters is still in flux, but here’s what we know, as outlined by the director of the Centers for Disease Control and Prevention and other top authorities:

What is the Biden administration recommending?

All Americans should get COVID-19 booster shots to bolster their protection amid the surging Delta variant and evidence that the vaccines’ effectiveness wanes over time. The plan is still awaiting a Food and Drug Administration evaluation of the safety and effectiveness of a third dose, the officials said.

But don’t only immunocompromised people need booster shots?

That was last week, when the FDA said organ transplant recipients and others with severely weakened immune systems could get an extra dose of the Pfizer or Moderna vaccines to better protect them as the Delta variant surges. But with COVID-19 patients again overwhelming some states, and the number of children hospitalized rising as the school year approaches, the Biden administration has rethought its position. Federal officials are especially alarmed by data from Israel indicating that the Pfizer vaccine’s protection against severe disease has fallen significantly for elderly people who received their second shot in January or February.

What happens next?

The Centers for Disease Control and Prevention said in a filing on Monday that it will convene its Advisory Committee on Immunization Practices on Aug. 24 for “discussions on additional doses of COVID-19 vaccine, including booster doses.” The CDC and the FDA will need to sign off on booster shots for the general population, as the agencies did last week for the far smaller number of immunocompromised patients.

Will patients need to get a booster dose of the same vaccine they initially received?

That remains to be seen. When the FDA cleared a booster dose of Pfizer and Moderna vaccines for people with compromised immune systems, the agency said recipients could get either one if their original vaccine wasn’t available. Both vaccines rely on messenger RNA technology to deliver a portion of the spike protein of the coronavirus to stimulate an immune response. (The FDA made no mention of immune-compromised patients who received the single-dose Johnson & Johnson vaccine.) It’s possible that regulators will make a similar recommendation for the great majority of vaccinated Americans.

Is anyone studying the safety and effectiveness of combining vaccines?

Yes. Several laboratory studies internationally have shown that combining vaccines that use different technologies can provoke immune responses that are even more robust than those triggered by multiple shots of the same vaccine. A laboratory study by the National Institute of Allergy and Infectious Diseases at 10 trial sites in the United States is now doing a head-to-head comparison of Pfizer, Moderna, and Johnson & Johnson booster doses on 450 fully vaccinated individuals; some will receive different shots than the ones they originally got. One of the trial’s principal investigators, Dr. Robert Atmar, of Baylor College of Medicine in Houston, said researchers hope to know in September whether mixing prime doses and booster doses is safe and generates high levels of antibodies. “It might provide some reassurance that it’s not necessary to try to match what a person received originally,” he said.

Does the Biden administration also expect to recommend booster doses for the Johnson & Johnson vaccine?

Yes. But regulators are waiting for the results of a clinical trial of two doses of Johnson & Johnson’s vaccine. Those results are expected later this month, according to Dr. Dan Barouch, who runs the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, which licensed technology for the Johnson & Johnson vaccine. That vaccine uses a different approach than the mRNA vaccines. It relies on a harmless and relatively rare cold virus, adenovirus serotype 26 — or Ad26 — to deliver part of the spike protein to trigger an immune response.

The World Health Organization recently criticized prosperous nations for proceeding with plans to provide boosters, saying it would be better to use the shots to inoculate people in poor countries with low vaccination rates. Have those concerns disappeared?

No. In fact, Dr. Lindsey Baden, an infectious-diseases specialist at Brigham and Women’s Hospital who helped lead the nationwide late-stage trial of Moderna’s vaccine, said Tuesday that he believes inoculating the unvaccinated in and outside the United States remains the most important challenge. “We will not address the variant problem until we vaccinate much larger parts of the world,” he said, noting that worrisome viral variants like Delta first emerged in countries with large swathes of unvaccinated people and spread elsewhere.

Baden said public officials should clarify their goals for deploying booster shots: Is it to prevent transmission of COVID-19 or to prevent severe cases that result in hospitalizations and deaths? States with high vaccination rates such as Massachusetts are seeing a rise in cases of the Delta variant, but the vaccines still seem to be preventing serious illnesses, he said, noting hospitals in Massachusetts are not being overrun.

“A year ago we had hundreds of inpatients,” Baden said, adding, “I’m not skeptical of the benefits of boosting, but we need to use it properly.”