People who got mixed doses of coronavirus vaccines – receiving a different vaccine type as a second dose than the first dose – appear to be more likely to experience mild side effects such as fever, chills, fatigue or headache, researchers in the UK reported Wednesday.
But the side effects following mix-and-match vaccinations were short-lived and there were no other safety concerns, the researchers reported in the Lancet medical journal.
“These are the type of reactions you do expect with vaccine,” Dr. Matthew Snape, an associate professor of pediatrics and vaccinology at the University of Oxford and chief investigator on the trial, said during a media briefing.
“They are more or less the same types of reactions that you’re seeing with the standard schedules. It’s just that they’re occurring more frequently, and we’re seeing both more frequent both in mild and moderate symptoms – but they resolved quickly,” Snape said.
Overall, “it’s a really intriguing finding,” he said, “and it’s not something necessarily we were expecting – to see such a consistent signal.”
It’s something to keep an eye out for when giving mixed doses, the researchers said.
“One of the things it’s telling us is that, for example, you wouldn’t want to immunize a ward full of nurses on the same day with a mixed schedule,” Snape said. “Because you may have higher rates of absenteeism in the next day.”
The mix-and-match trial
The new research included 830 volunteers 50 and older who were randomly assigned to four different vaccine schedules involving the Oxford/AstraZeneca and Pfizer/BioNTech vaccines, with first and second doses given 28 days apart.
They either got the AstraZeneca vaccine as both doses; AstraZeneca as a first dose and Pfizer as a second dose; the Pfizer vaccine as both doses; or the Pfizer vaccine as a first dose and AstraZeneca as a second dose.