Angela Rasmussen, a virologist at Georgetown University, received a booster shot of Pfizer and BioNTech's Covid-19 vaccine in late June, two months after she got Johnson & Johnson's single dose.

Other countries, such as Italy, are also allowing those under the age of 60 who received a first dose of AstraZeneca's vaccine to get a different shot when they get their second dose.

Executives from Pfizer, Moderna and J&J have said they expect Americans will need booster shots, and Pfizer has said it plans to ask the FDA to authorize boosters as it sees signs of waning immunity.

Since Rasmussen received her booster shot, a new study has suggested the J&J vaccine is much less effective against the delta and lambda variants than against the original virus.

The researchers who led the study, which has not yet been peer-reviewed, now say they hope J&J recipients will eventually receive a booster shot of the Pfizer or Moderna vaccines.

"I would guess that those who received a single dose of Johnson & Johnson may need a booster of an mRNA vaccine than other people need boosters," said Dr.

The trial will include adults who have received one of the three Covid vaccine regimens currently available in the U.S.: J&J, Moderna or Pfizer.

Another study, published in Nature Medicine on Monday, found adding the Pfizer or Moderna vaccines as a second dose to J&J led to a better immune response than just two doses of the J&J vaccine and was well tolerated.

"The reason for this is because if you're using a vectored vaccine, like the J&J or AstraZeneca, which use adenovirus, you generate antibodies against the vector so that your second shot with the same vector makes it less likely to induce a robust immune response," she said.

Aside from Covid, mixing and matching vaccines that use different platforms is not something unusual, Iwasaki said.

Mixing vaccines may prevent the body from causing a sort of "mediated clearance of the vaccine itself," where the shot is less effective, Iwasaki said.

"The spike protein that is used for the J&J and Moderna and Pfizer vaccines are virtually identical, so it is not like you're getting cross-reactive protection against different variants," she said.

Soumya Swaminathan, recently advised individuals against mixing and matching Covid vaccines from different manufacturers, saying the practice is a "data-free zone" and immunogenicity and safety both still need to be evaluated further.

Still, some doctors are already suggesting immunosuppressed populations, such as patients living with cancer or those who have had organ transplants, might benefit from an extra dose of the same or another Covid vaccine, Barouch said.

A CDC advisory group on Thursday considered whether fully vaccinated Americans with weakened immune systems need a booster dose of a Covid vaccine after data shows they are less likely to have antibodies to fight the disease and more likely to suffer from a so-called breakthrough infection.

Paul Offit, who advises the FDA on Covid vaccines, said the CDC should provide "more direction" and create guidelines for how providers should be thinking about mixing and matching vaccines