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Study: Mixing COVID vaccines causes side effects

Published: May. 14, 2021 at 2:10 PM EDT|Updated: May. 14, 2021 at 2:11 PM EDT
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(CNN) – When it comes to the Moderna and Pfizer COVID-19 vaccines, the Centers for Disease Control and Prevention has long said the two are not interchangeable and shouldn’t be mixed.

It means if you start with one, you don’t finish with the other. Both doses should be from the same manufacturer.

And now, a recent study suggests there may be even more of a reason to not blend vaccines.

According to researchers in the United Kingdom, doing so appears to make it more likely for a person to experience mild side effects.

“They are more or less the same types of reactions that you’re seeing with the standard schedules,” said Dr. Matthew Snape, the chief trial investigator. “It’s just that they’re occurring more frequently, and we’re seeing both more frequent, both in mild and moderate symptoms.”

Those symptoms include fever, chills, fatigue and headache.  Researchers say those side effects were short-lived.

No other safety concerns were found.

“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 because you may have higher rates of absenteeism in the next day,” Snape said.

The study involved the Pfizer and AstraZeneca vaccines. AstraZeneca has not been authorized for use in the United States.

The UK researchers are now testing mixed administration of vaccines made by Moderna and Novavax.

The Novavax vaccine also has not been authorized for use in the United States.

The study says more research is needed on immune system responses when different vaccines are used in the same schedule.

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