This comes after researchers presented findings from a National Institutes of Health “mix and match” study—which looked at how COVID-19 vaccines and boosters from Moderna, Pfizer, and Johnson & Johnson worked in tandem with each other—to an FDA advisory panel on Oct. 15. The findings showed that recipients of the one-dose Johnson & Johnson vaccine produced stronger antibody levels after receiving either a Pfizer or Moderna booster shot, compared to a J&J booster. The study also found that those who originally took either the Pfizer or Moderna vaccine produced comparably strong immune responses after receiving either of the company’s booster shots.

A panel of advisors to the Centers for Disease Control will give on Thursday its own recommendations on boosters. The CDC is expected to agree with the FDA’s decision and encourage flexibility when it comes to mixing vaccines. If both agencies agree, boosters could be available for Americans who qualify by Friday.

The NIH study, which has not yet been peer-reviewed and had a small sample size, found that those who originally received the Johnson & Johnson vaccine produced stronger antibody levels after receiving booster shots made by Moderna or Pfizer, compared to boosters from Johnson & Johnson. The FDA did not recommend any vaccine over the other, and also said nothing on whether it’s preferable to use the same vaccine. Vaccine providers, however, would have discretion to offer Americans boosters of a different vaccine brand than they originally received, reports The New York Times.

Some European countries have already allowed people to take a mix-and-match approach. Back in January, the U.K. authorized mixing and matching for those who did not have access to a second dose of the vaccine they originally received. A preliminary British study showed that individuals who received one dose of the Pfizer vaccine and one dose of the AstraZeneca vaccine produced high levels of antibodies and a protective immune response against COVID-19.

Health officials had been asking for a mix-and-match approach for weeks, believing that patients should have a choice over what booster they receive—either because of adverse reactions to their initial shots or because they are more confident in the immune response a particular brand will provoke. Providers could also have limited supply of certain boosters.

The FDA also officially authorized booster shots for recipients of the Moderna and Johnson & Johnson vaccines on Wednesday, a month after their approval of the Pfizer-BioNTech booster.  The FDA’s panel of advisers recommended the Johnson & Johnson booster for people 18 and older who received their initial immunization at least two months earlier. On the other hand, the panel only recommended the Moderna booster for people at least 65 years old and younger adults who risk severe illness or viral exposure at work—the same groups the FDA and CDC authorized the Pfizer booster for last month. Those eligible for the Moderna and Pfizer boosters also need to have received their last dose of the original vaccine at least six months before receiving the booster.

Different eligibility requirements for the three different booster shots could make mixing and matching more complicated and confusing. For now, the authorization of a mix-and-match approach seems most geared toward allowing recipients of the single-dose Johnson & Johnson vaccine to have options with regards to what booster they receive. But experts say any of the boosters offer good protection against severe disease and death.

Update, October 21, 2021: This article has been updated with news about the FDA authorization of boosters.

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• COVID-19 vaccine makers could inoculate the world by the end of 2022, says J&J CEO Alex Gorsky
• Got the J&J vaccine? Here’s what you need to know about boosters