It is likely safe and effective, but investigators are still gathering information to be sure.

The authorized COVID-19 shots across the globe are all designed to stimulate your immune system to produce virus-fighting antibodies, although 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 the mix-and-match regimens will work,” she said.

Researchers at Oxford University in the United Kingdom are examining combinations of their two-dose COVID-19 vaccines produced by AstraZeneca, Moderna, Novavax and Pfizer-BioNTech. Smaller trials are also continuing in Spain and Germany.

“We really just have to receive the evidence in each of them (vaccine) combinations,” O’Brien said.

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

That is because mixing and matching different kinds of vaccines can often produce a more powerful immune response, stated Lawrence Young, a virologist at the University of Warwick in the uk.

In some areas, health officials already suggest mixing in select conditions.

Following the AstraZeneca vaccine was linked to exceptionally infrequent blood clots, many European countries such as Germany, France and Spain recommended people who got it as a first dose get a Pfizer or Moderna shot as another dose instead.

In Britain and Canada, officials say people should aim to receive exactly the same vaccine for their next dose if at all possible. If they got AstraZeneca as their first injection, they’re advised to get another vaccine only if they have a history of blood clots or other conditions which may place them at higher risk of clot.