WASHINGTON (AP/KDKA) — Federal regulators are expected to authorize the mixing and matching of COVID-19 booster doses this week in an effort to provide flexibility as the campaign for extra shots expands.

The upcoming announcement by the Food and Drug Administration is likely to come along with authorization for boosters of the Moderna and Johnson & Johnson shots and follows the OK for a third dose for the Pfizer vaccine for many Americans last month. The move was previewed Tuesday by a U.S. health official familiar with the matter who was not authorized to speak publicly ahead of the announcement.

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The FDA was expected to say that using the same brand for a booster was still preferable, especially for the mRNA vaccines from Pfizer and Moderna that have proved most effective against the coronavirus. The agency was still finalizing guidance for the single-shot J&J vaccine.

“When you’re talking about mixing and matching you’re talking about mixing two different technologies. So the studies have been done on the Johnson & Johnson vaccine followed by an mRNA vaccine because those take advantage of two different technologies. And there have been people trying to improve the effectiveness of the J&J vaccine and this is one strategy,” said infectious disease expert Dr. Amesh Adalja.

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Preliminary results from a government study of different booster combinations found an extra dose of any type revs up levels of virus-fighting antibodies regardless of the brand people first received. But recipients of the single-dose J&J vaccination had the most dramatic response — a 76-fold and 35-fold jump in antibody levels, respectively, shortly after either a Moderna or Pfizer booster, compared to a four-fold rise after a second J&J shot.

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“There may be a difference in the order. Meaning that people who got Moderna and Pfizer vaccines probably don’t get the same boost as someone who got it the other way around where you get Johnson & Johnson and then switched to Moderna or Pfizer. This probably has to do with the nuances with the immune system and the differences with the vaccine technologies,” said Adalja.

One confusing decision is what Moderna dose to recommend in combination with other brands. Moderna has applied for its booster to be half the original dose, saying that’s plenty for people who already received two full-strength shots. But the mix-and-match study used full-strength extra doses, and there’s no way to know if a half-dose Moderna booster would trigger as strong a reaction in J&J recipients.

Allowing mixing and matching could make the task of getting a booster simpler for Americans and allow people who may have had adverse reactions to the initial dose to try a different shot.

Last week, the U.S. said it would recognize combinations of vaccines administered overseas for the purposes of entering the country. The practice was common in Canada and some European countries in the early months of the vaccination campaign.

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