The Food and Drug Administration is expected to allow giving Covid-19 vaccine booster shots that are a different vaccine type from the one used in initial doses, two sources familiar with the matter said.
The sources confirmed reporting by The New York Times, which reported Monday that the FDA was planning the move.
The approach is sometimes called "mix and match." A National Institutes of Health study found the approach to be safe and effective.
The sources said Tuesday that an announcement could come as soon as Wednesday.
State health officials have been asking for such flexibility since late last month, when the FDA authorized the Pfizer-BioNTech booster shot, to be given at least six months after an initial series.
Mix and match will facilitate vaccination clinics at places like nursing homes. Instead of requiring vaccine providers to bring doses from all three companies, providers can bring what they have in stock, according to the Association of State and Territorial Health Officials.
A source said the process needs to move forward with urgency to give doctors flexibility and because some people have already started getting booster shots that are different from their initial rounds of vaccinations.
The Times reported that the government would not recommend one type over another and that it could still say it is preferable that booster shots use the same vaccine as first vaccinations.
Allowing people who got the Johnson & Johnson vaccine to get Pfizer or Moderna booster shots means that the demand for the J&J vaccine could decline even more and that existing doses on states' shelves could end up going to waste, state health officials say.
The NIH study about mixing and matching found that people who received the Johnson & Johnson vaccine produced stronger antibody levels after they got booster shots made by Moderna or Pfizer, compared to boosters from Johnson & Johnson.
People who were originally vaccinated with the Pfizer or Moderna vaccines and got either company's booster shot produced similarly strong immune responses, the researchers found.
Dr. Ashish Jha, the dean of the Brown University School of Public Health, said Tuesday on NBC's "TODAY" show that reports that the FDA could allow a mix-and-match approach was good news.
"Clinically, all the data says that mixing and matching is just fine, there's no downside," Jha said. He said it could also help in logistics, because people would not have to stick to the same type of shot.
More than 104 million of the people who have been fully vaccinated against Covid-19 in the U.S. got the Pfizer vaccine, according to the Centers for Disease Control and Prevention.
More than 69 million people got the Moderna vaccine, and more than 15 million got the Johnson & Johnson vaccine, according to the CDC.