Covid-19 vaccination: Here's what will happen if you mix two vaccine shots

Mixing of doses of two leading coronavirus vaccines raised side effects for patients such as fatigue and headaches, early findings from a study show. People who were administered a first dose of AstraZeneca's vaccine followed by Pfizer's vaccine four weeks later experienced more short-lived side effects, researchers from the University of Oxford reported in The Lancet medical journal.

That also stood true when the order of the vaccines was changed, according to a Bloomberg report.

Vaccine scarcity in many low and middle-income nations has prompted researchers and public health officials to examine strategies such as mixing two different shots. If mixing of two vaccines is proved safe and effective, it would make it easier for the governments to use their stockpiles efficiently.

In France, people who received a first dose of the Astra vaccine before it was assigned to older patients, have been offered the vaccine made by Pfizer and BioNTech SE for their second injection, the report said.

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"It's a really intriguing finding and not something that we were necessarily expecting," said Matthew Snape, an Oxford pediatrics and vaccinology professor who's leading the trial. "Whether or not this will relate to an improved immune response, we don't know yet; we'll be finding out those results in a few weeks' time.

The study didn't signal any safety issues and the stronger side effects vanished after a few days, he said on a conference call.

However, mixed dose schedules could lead to an increase in work absences the day after immunisation, according to Snape.

"You wouldn't want to immunise a ward full of nurses with a mixed schedule on the same day," he said Severe fatigue was reported in about 10% of participants who received mixed doses compared with about 3% for those who got a single type of vaccine, the research shows.

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The study comprised participants aged 50 and above. It's possible that the reactions could be even stronger in younger patients, said Snape.

The researchers are also testing a wider interval of 12 weeks between shots and its effects on the participants.

Also, they plan to expand the research to include vaccines from Moderna Inc. and Novavax Inc.

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