Can you mix and match two-dose COVID-19 vaccines?

It's likely safe and effective, but researchers are still gathering data to be sure.

The authorized COVID-19 shots around the world are all designed to stimulate your immune system to produce virus-fighting antibodies, though the way they do so varies, noted Dr. Kate O'Brien, director of the World Health Organization's vaccine unit.

"Based on the basic principles of how vaccines work, we do think that the mix-and-match regimens are going to work," she said. 

Scientists at Oxford University in the United Kingdom are testing combinations of the two-dose COVID-19 vaccines made by AstraZeneca, Moderna, Novavax and Pfizer-BioNTech. Smaller trials are also ongoing in Spain and Germany.

"We really just need to get the evidence in each of these (vaccine) combinations," O'Brien said.

So far, limited data suggests an AstraZeneca shot followed by the Pfizer shot is safe and effective. The combination also appears to come with a slightly higher likelihood of temporary side effects like aches and chills.

That might be because mixing and matching different types of vaccines can often produce a stronger immune response, said Lawrence Young, a virologist at the University of Warwick in the United Kingdom.

In some places, health officials already suggest mixing in select circumstances.

After the AstraZeneca vaccine was linked to extremely rare blood clots, several European countries including Germany and France recommended people who got it as a first dose get a Pfizer or Moderna shot as a second dose instead. On Thursday, Canada made the same recommendation.

Some places like Britain say people should aim to get the same vaccine for their second dose if possible. If they got AstraZeneca as their first shot, they're advised to get another vaccine only if they have a history of blood clots or other conditions that might put them at higher risk of clots.