Health officials say both doses should be from the same vaccine.
The COVID-19 vaccines rolling out in the US, UK and other parts of the world so far require two shots a few weeks apart.
In the United States where the Pfizer and Moderna vaccines are distributed, health officials say the vaccines are not interchangeable. In England where footage is available from Pfizer and AstraZeneca, officials also say dosing should be consistent.
But in the rare cases where the same type is not available or if it is not known what was administered in the first shot, English officials say it is acceptable to give any available vaccine for the second episode. Because the Pfizer and AstraZeneca vaccines work in a similar way, they say an unmatched dose is better than partial protection.
Without any studies, vaccine doses should not be mixed, Naor Bar-Zeev, a vaccine expert at Johns Hopkins University, said.
If people happen to get a different vaccine for the second dose by chance, Bar-Zeev said it is likely “to work well and likely to be well tolerated”, but evidence is needed to confirm.
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