A medical assistant administers a COVID-19 vaccine dose to a woman at a clinic in Los Angeles on March 25, 2021.
  • Mixing Pfizer and AstraZeneca doses yields a greater immune response than two doses of AstraZeneca.
  • That's according to early data from a UK study, though it hasn't been peer reviewed yet.
  • One expert thinks it will be beneficial to get an mRNA booster after J&J's single shot.
  • For more stories visit Business Insider.

The evidence in favour of a mix-and-match approach to coronavirus shots is growing.

Early data from a UK study, though not yet been peer reviewed, indicates that mixing the Pfizer and AstraZeneca vaccines yields a greater immune response than two doses of AstraZeneca's shot. The researchers measured antibody levels among 830 volunteers who received either two doses of the same vaccine or a combination of Pfizer and AstraZeneca, four weeks apart.

Volunteers who got AstraZeneca's shot followed by Pfizer's showed nearly the same antibody levels - and an even higher number of T-cells - as those who'd received two doses of Pfizer. By contrast, volunteers who got Pfizer's shot first and AstraZeneca's second had slightly lower antibody levels than the reverse combination. But those levels were still were roughly five times higher than the antibody response after two doses of AstraZeneca.

"Mixing these two vaccines not only appears to be safe, but can give a higher immune response than the standard dosing regimens," Deborah Dunn-Walters, a professor of immunology at the University of Surrey, said in a statement.

There are some caveats, though: The UK study examined immune-system responses, not real-world infection rates. Plus, research has shown that AstraZeneca's vaccine is more effective when the doses are given 12 weeks apart. So the UK team is running a separate trial based on that interval, and those results haven't been released yet.

For now, though, some experts think it might be beneficial to recommend a booster shot of Pfizer's or Moderna's mRNA vaccine for people who received a different initial shot.

"From this study, people who have had a first course of AstraZeneca should probably be offered the Pfizer vaccine (or possibly Moderna or Novavax dependent on the future trial data from this study) in the autumn rather than a repeat AstraZeneca," Paul Hunter, a medicine professor at the University of East Anglia, said in a statement.

"People who had a Pfizer first course may not need an autumn booster," he added, "but if they do, then it probably does not matter much which vaccine they are offered."

Some countries already let residents mix and match

Chanei Henry, senior research coordinator of molecular virology and microbiology at Baylor College of Medicine, prepares a COVID-19 vaccine.

Even before coronavirus vaccines came out, some research had already indicated that mixing and matching vaccines in general could yield better results. Johnson & Johnson's Ebola vaccine, for instance, uses an adenovirus shot followed by a modified version of a poxvirus. Researchers have tested different vaccine combos for a future HIV shot, too.

Already, some countries have started allowing residents to mix and match coronavirus vaccines.

Canada announced earlier this month that the Pfizer and Moderna shots could be used interchangeably, given the country's low vaccine supply. Meanwhile, several European countries - including Germany, Sweden, France, Spain, and Italy - have said that people who received an initial dose of AstraZeneca's shot can choose a different vaccine for their second dose, given the reports of rare blood clots linked to AstraZeneca's vaccine.

But there's still the question of side effects. Volunteers in the UK study who got a combination of Pfizer and AstraZeneca reported higher rates of chills, headaches, and muscle pain than people who'd received two doses of the same shot. However, a recent Spanish trial found that one shot of AstraZeneca's vaccine followed by a Pfizer dose yielded mostly mild or moderate side effects.

Yet another mix-and-match study, this one funded by the National Institutes of Health, is giving participants a booster of Moderna's vaccine after their initial regimen of Moderna, Pfizer, or Johnson & Johnson.

Joseph Hyser, a participant in that trial, recently told Insider that his side effects were a bit more severe after a booster of Moderna's vaccine than after each of his doses of Pfizer's vaccine. He said he woke up the day after his booster feeling "like I had done a very rigorous gym workout," with chills and arm soreness - like he'd gotten a "hard punch in the shoulder."

The NIH study could offer much-awaited insight into whether following J&J's single-dose shot with a Moderna booster offers similar protection to a two-dose mRNA vaccine. But some experts who got J&J's shot aren't waiting. Angela Rasmussen, a virologist at the University of Saskatchewan, tweeted on Tuesday that she got the Pfizer vaccine to "top off" the J&J vaccine she received in April.

Rasmussen encouraged people who'd had J&J's shot to speak with healthcare providers about getting an extra dose.

"We shouldn't wait to make recommendations about this," she wrote.

Hilary Brueck and Dr. Catherine Schuster-Bruce contributed reporting.

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