It's still not recommended in the US, but new data may shed light on the safety and effectiveness of a mix-and-match approach.

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Certain older and at-risk individuals who received Pfizer's COVID-19 vaccine are beginning to line up for Pfizer booster doses. But if you're among the millions of fully vaccinated Americans who received Moderna or Johnson & Johnson (J&J), you might be wondering, Where's my booster dose?

Is it OK to get one brand's vaccine and a booster dose of another?

Initial results of a mix-and-match booster trial by the National Institutes of Health may provide some answers—but we'll have to wait for those results. During a September 28 media briefing, White House Chief Medical Advisor Anthony Fauci, MD, confirmed that trials involving initial vaccination with one COVID vaccine followed by a booster dose of another product are ongoing.

Data on Moderna as the booster dose to all three vaccines are already available, Dr. Fauci told reporters. J&J data will be available in a matter of days, and Pfizer data are anticipated in the first week or two of October, he added. Of course, all of that data must be submitted to the US Food and Drug Administration (FDA) for regulatory approval. "So you don't want to get ahead of the FDA, but at least that's where the data are right now," he said.

At present, the US Centers for Disease Control and Prevention (CDC) says COVID-19 vaccines are not interchangeable. But as scientists and clinicians consider how best to get people fully vaccinated—and boosted when necessary—against the highly transmissible Delta variant, the question arises: Is it OK to mix vaccines? Here's what we know so far.

What does it mean to 'mix' vaccines?

The concept is simple, really. Mixing vaccines can mean one of two things: getting an initial dose of one manufacturer's COVID-19 vaccine followed by a second dose from another manufacturer a few weeks later, or getting a one- or two-dose vaccination from one brand and a booster from another.

To date, data on the safety and effectiveness of mixing products has been limited. And that's why CDC does not yet endorse a mix-and-match approach.

Why would someone want to mix vaccines?

There are a number of reasons for mixing vaccines. In parts of the world where the shots are in short supply, the ability to mix vaccines from difference manufacturers "may reduce the pressure on vaccine supply," according to GAVI, an international, public-private alliance that makes vaccines available to children around the world.

There's also the question of what works best. GAVI points to preliminary evidence indicating that two doses from different vaccine makers may elicit a stronger immune response than two jabs of the same vaccine. Indeed, early study results out of the UK suggest a mix-and-match approach—one dose of AstraZeneca (which is not available in the US) and one dose of an mRNA vaccine (in this case, Pfizer)—produced a better immune response than two AstraZeneca doses.

"We're noticing that the levels of protection are equivalent to as though you got two doses of the mRNA vaccine," Vin Gupta, MD, affiliate assistant professor of health metrics sciences at the University of Washington's Institute for Health Metrics and Evaluation in Seattle, told Health during an interview in June.

Earlier in the year, concerns about vaccine side effects drove some decisions about mixed dosing. Some European countries halted use of the AstraZeneca vaccine after reports linking it to a rare blood clotting disorder (mostly in younger adults), leaving many people partially vaccinated, Nature reported.

In turn, government officials in France recommended reserving the vax for people 55 and older and allowed younger adults who had an initial dose of AstraZeneca to receive a second dose of an mRNA vaccine.

Are other countries mixing vaccines?

While the US has not yet followed suit, some counties, like Germany, have embraced a mix-and-match approach, according to William Schaffner, MD, professor of infectious diseases at Vanderbilt University Medical Center in Nashville.

"People who've received the first dose of AstraZeneca are permitted and, indeed, encouraged to receive a second dose of an mRNA vaccine," he previously told Health. Those people include Dr. Schaffner's own son, who lives in Germany and received an AstraZeneca dose followed by Pfizer.

Canada, Italy, Spain, and Sweden also allow people to receive a different vaccine for their second dose, the New York Times reported.

While AstraZeneca is not an option in the US, it uses the same technology as the Johnson & Johnson/Janssen vaccine (J&J) to activate the body's immune response to the virus. Both AZ and J&J are adenovirus vector, aka "viral vector," vaccines.

Also important to note: While the CDC advises against mixing and matching, it does address those "exceptional situations" in which it's unclear whether someone had an initial dose of a Pfizer or Moderna mRNA vaccine, for example. In that case, "any available mRNA COVID-19 vaccine may be administered" to complete the series.

How safe is it to mix vaccines?

Could combining two different vaccines lead to more or different adverse effects? Per the recent Nature report: "No mix-and-match trials have yet reported severe side effects."

But that's something researchers are trying to tease out. For now, the public health guidance remains unchanged: If you get a J&J shot, you only need one to be considered fully vaccinated, and if you go for the mRNA vaccine, you need two, per the CDC.

As for boosters, certain Pfizer vaccine recipients may be eligible for a Pfizer booster dose, but if you got Moderna or J&J, sit tight. More data are expected soon, and armed with the info, CDC says it will keep the public informed on its plan for booster shots.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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