The Food and Drug Administration is planning to allow Americans to receive a different Covid-19 vaccine as a booster than the one they initially received, a move that could reduce the appeal of the Johnson & Johnson vaccine and provide flexibility to doctors and other vaccinators.

The government would not recommend one shot over another, and it might note that using the same vaccine as a booster when possible is preferable, people familiar with the agency’s planning said. But vaccine providers could use their discretion to offer a different brand, a freedom that state health officials have been requesting for weeks…

While the research on mixing and matching doses is somewhat thin, even some scientists who have strongly criticized the Biden administration’s booster policy said that providers should be given a measure of discretion as the campaign ramps up.

“If you look at the data, it certainly looks like it might be better,” Dr. Paul A. Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said of Moderna or Pfizer boosters for Johnson & Johnson recipients. “I think we should move quickly on this, because it’s already happening.”