The U.S. Food and Drug Administration is planning to allow Americans to mix and match COVID-19 vaccines when getting a booster shot, according to reporting by The New York Times and The Washington Post.

By way of example, those who received two doses of the Pfizer vaccine can get the Moderna or Johnson & Johnson vaccine as their booster shot. And those who got the single-dose J&J shot can turn to Moderna or Pfizer for their booster. Here are a few things to know about mixing and matching vaccines.

Let’s talk about the COVID boosters first. What has the FDA advisory panel said about them?

An independent panel that advises the FDA has recommended a half-dose of Moderna’s COVID-19 vaccine as a booster for people 65 and older, as well as adults with other health problems, jobs or living situations that put them at increased risk for COVID-19.

The FDA panel has endorsed boosters of Johnson & Johnson’s single-shot COVID-19 vaccine for people 18 and older. J&J has asked the FDA for flexibility, arguing that the extra dose adds important protection as early as two months after initial vaccination — but that it might work better if people wait until six months later, the Associated Press reported.

The FDA and the Centers for Disease Control and Prevention have already recommended the Pfizer booster for similar groups for at least six months after the second dose. The FDA and CDC will likely rule on the Moderna and J&J boosters this week. With federal authorization, the three booster vaccines would become available for millions of Americans.

The FDA may also say that people should generally stick to the same vaccine if possible, two federal officials told The Washington Post.

What did the FDA advisory panel say about mixing and matching vaccines?

The panel of independent vaccine experts considered on Friday the safety and benefits of “mixing and matching” vaccines — for example, taking a Pfizer booster shot after receiving the Moderna or Johnson & Johnson vaccine.

FDA’s advisers didn’t vote on whether that should be recommended but told the government to allow flexibility with boosters, saying there were no safety red flags even if it’s not yet clear just how much difference, if any, mixing and matching may make in long-term protection, the AP reported.

Dr. Angelica Cifuentes Kottkamp, an assistant professor of medicine at NYU Langone Health in New York, told The Dallas Morning News last week that with these results, doctors and researchers can say with confidence that these boosters are safe and effectively produce an immune response regardless of the vaccine regimen received originally.

Mixing and matching vaccines may also boost waning immunity and expand the breadth of immunity against variants, she said.

One issue that may affect the decision about how to use boosters is the association of the J&J vaccine with a very rare clotting side effect that tended to be more common in women under 50, The Washington Post reported. Health care providers could recommend a messenger RNA dose -- Pfizer or Moderna -- as a second shot to people who may be at increased risk for an adverse event.

What do preliminary studies indicate about mixing and matching vaccines?

Preliminary results from an ongoing National Institutes of Health study of different ways to mix and match different shots showed that a booster of any sort revved up people’s levels of antibodies — at least for a few weeks. And the most dramatic jump came from giving a Pfizer or Moderna shot after the single-dose J&J vaccination, the AP reported.

The NIH study found that people who got the J&J shot who received a Moderna booster saw their antibody levels rise 76-fold in 15 days, compared with only a fourfold increase after an extra dose of J&J, The New York Times reported.

A shot of the Pfizer vaccine also raised the antibody levels of J&J recipients more than a J&J booster did, although not as much as Moderna did.

Experts cautioned last week that the new data was based on small groups of volunteers and short-term findings. The study’s researchers warned against using the findings to conclude that any one combination of vaccines was better, The New York Times reported.

As the NIH’s clinical trial continues, the researchers are studying the immune responses to multiple variants, Kottkamp of NYU Langone Health told The News.

“Mixing vaccines in a safe way should allow other nations to prioritize products and reach vaccination goals,” she said. “However, to get a better answer and to be totally equal, we should try to test combinations that include other vaccines used in the world, not only those authorized in the [U.S.]”

Meanwhile, University of Oxford researchers are studying the effects of mixing and matching doses of the Pfizer and AstraZeneca vaccines, The New York Times reported.

Preliminary results indicate that the Pfizer-AstraZeneca combination has been well tolerated, Kottkamp said.