The COVID-19 vaccines available so far require two shots, and health officials say the doses should be the same kind. In the U.S., health officials say the vaccines are not interchangeable.

Officials in England also say the doses should be consistent. In the rare event that the same kind isn’t available, English officials say it’s OK to give whichever vaccine is available for the second shot. They say a mismatched dose is better than partial protection.

But without any studies, vaccine doses should not be mixed, said Naor Bar-Zeev, a vaccine expert at Johns Hopkins University.

If people do happen to get a different vaccine for their second shot by accident, Bar-Zeev said it is likely “to work fine and likely to be well tolerated,” but evidence is needed to be sure.