Mixing COVID Vaccines Might Raise Odds for Minor Reactions: Study

THURSDAY, May 13, 2021 (HealthDay News) -- Mixing the various COVID-19 vaccines -- for example, getting a first dose of the Pfizer vaccine but having your second dose be the AstraZeneca shot -- seems to increase the risk of side effects, a new study from Britain suggests.

Preliminary data from a study of 850 U.K. patients aged 50 and older that compared mixed dosing schedules of the Pfizer and AstraZeneca vaccines found an increase in the frequency of mild to moderate symptoms in people receiving mixed doses. The AstraZeneca vaccine is not available in the United States.

The adverse reactions didn't last long and there were no other safety concerns, the researchers stressed. In areas where the supply of one vaccine is limited, experts have wondered if the "mix-n-match" approach might work.

"Whilst this is a secondary part of what we are trying to explore through these studies, it is important that we inform people about these data, especially as these mixed-doses schedules are being considered in several countries," said researcher Matthew Snape, an associate professor in pediatrics and vaccinology at the University of Oxford.

"The results from this study suggest that mixed dose schedules could result in an increase in work absences the day after immunization, and this is important to consider when planning immunization of health care workers," he said in a university news release.

"Importantly, there are no safety concerns or signals, and this does not tell us if the immune response will be affected. We hope to report these data in the coming months. In the meantime, we have adapted the ongoing study to assess whether early and regular use of paracetamol [acetaminophen] reduces the frequency of these reactions," Snape said.

The researchers also said these data were from patients 50 and older, and it's possible these reactions may be more common in younger patients.

The trial is being done to see if mixing these vaccines might be an effective way of protecting people from COVID-19, especially if booster doses are needed.

In the study, four different combinations of prime and booster vaccination were given: a first dose of the AstraZeneca vaccine followed by boosting with either the Pfizer vaccine or a further dose of the AstraZeneca vaccine, or a first dose of the Pfizer vaccine followed by boosting with either the AstraZeneca vaccine or a second dose of the Pfizer vaccine.

In April, the researchers started to evaluate the Moderna and Novavax vaccines in a new U.K. study. (The Novavax vaccine is going through the approval process in the United States and Britain.) Volunteers received either the AstraZeneca or Pfizer vaccine, and were then randomly allocated to receive either the same vaccine for their second dose or a dose of the vaccines produced by Moderna or Novavax.

The report was published May 12 in the journal The Lancet.

More information

For more on COVID-19 vaccines, see the U.S. Centers for Disease Control and Prevention.

SOURCE: University of Oxford, news release, May 12, 2021