Doing This With Your Second Shot Leads to More Side Effects, Study Says

You may have a more intense reaction to the COVID vaccine if you do this.

By now you know that the Pfizer-BioNTech and Moderna COVID vaccines are given in two doses, three and four weeks apart respectively. If you're one of the many people across the U.S. who has already received your first COVID jab—which the Centers for Disease Control and Prevention (CDC) estimates to be 59 percent of adults as of May 13—and you're waiting for your second dose, you'll want to know the results of a new study. There's been a lot of discussion about combining two different brands of vaccines, something the CDC advises against unless it's a "limited, exceptional situation." And now, there's proof that it may not be the best idea. The Oxford Vaccine Group's Com-Cov vaccine trial found that mixing COVID vaccine types can lead to more non-severe vaccine side effects.

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The University of Oxford led-study included 463 volunteers who were at least 50 years old. The researchers randomly assigned the participants to receive either two of the same shot or a combination of two vaccines, both taken 28 days apart. The volunteers were split into four groups: Some initially received the AstraZeneca-Oxford COVID vaccine, which has already been approved for use in the U.K., and then Pfizer-BioNTech second. Others did the opposite, Pfizer first and AstraZeneca second. A third group took two doses of the Pfizer vaccine and a fourth was assigned two shots of the AstraZeneca vaccine.

Ultimately, the results, which were published in The Lancet, found that participants were more likely to report mild and moderate vaccine side effects—headaches, chills, and muscle pain—after receiving two different doses, CNN reports.

The findings noted that 34 percent of participants who got the AstraZeneca jab first and Pfizer shot second reported feverishness, while only 10 percent of volunteers had fevers after getting two doses of AstraZeneca. Additionally, 41 percent of the participants who received the Pfizer jab first and AstraZeneca shot second reported a fever. However, only 21 percent of the people who received two Pfizer shots experienced that side effect. According to the findings, most reactions cropped up within 48 hours of the second dose. Not one participant experienced a blood clot within seven days of immunization.

As a result, the authors note that this mixed approach to vaccinations "might have some short-term disadvantages."

A healthcare worker wearing gloves fills a syringe with COVID-19 vaccine

"These are the type of reactions you do expect with vaccine," Matthew Snape, MD, an associate professor of general pediatrics and vaccinology at the University of Oxford and chief investigator on the trial, said at a media briefing, according to CNN. "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 added: "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."

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When the researchers started the study, Snape said in a statement: "If we do show that these vaccines can be used interchangeably in the same schedule this will greatly increase the flexibility of vaccine delivery, and could provide clues as to how to increase the breadth of protection against new virus strains."

The Oxford team has now turned to studying how the Moderna jab and Novavax shot, a vaccine which is still in the trial phase, work when mixed. And more research is needed to learn how combining vaccines can affect the body's immune response.

Research like this is important because, as Jeremy Brown, a member of the U.K.'s Joint Committee of Vaccination and Immunization, says, people may very well need a mix of COVID vaccine doses in the future. "It's practically going to have to be that way because, once you've completed a course of, say, the Moderna or Pfizer or the AstraZeneca, with two doses—in the future, it's going to be quite difficult to guarantee you get the same type of vaccine again," he told the BBC in April.

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