There is currently no evidence regarding whether it is safe to mix and match coronavirus vaccines. So far the general consensus among health officials has been that both doses of the COVID-19 vaccine should be of the same vaccine and they should not be mixed.

The COVID-19 vaccines rolling out in the U.S., the U.K., Turkey and other parts of the world so far require two shots given a few weeks apart. Mixing has become the second main contention point about mass vaccinations after the waiting period between both doses. (For more Q&As on COVID-19 vaccines, click here.)

In the U.S., where the Pfizer-BioNTech and Moderna vaccines are being distributed, health officials say the vaccines are not interchangeable. In England, where shots by Pfizer and AstraZeneca are available, officials also say the doses should be consistent.

But in the rare event that the same kind isn’t available or if it’s not known what was given for the first shot, English officials say it’s OK to give whichever vaccine is available for the second shot. Since the Pfizer-BioNTech and Oxford-AstraZeneca vaccines focus on the spike protein that coats the coronavirus, they say a mismatched dose is better than partial protection.

Neither the U.S. nor Turkey has expressed support for this move and have stuck to administering one vaccine only so far. Turkey has also ordered 50 million doses of CoronaVac, the inactive virus vaccine developed by China's Sinovac, which operates on a different principle than mRNA vaccines.

Nevertheless, 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.