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I got the Moderna vaccine. Can I get a Pfizer booster?

Here's what's known about mixing-and-matching Covid-19 vaccines.
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Efforts to give a Covid-19 booster shot to certain adults in the United States have officially begun, following a week of intense deliberation from government advisory committees

Early Friday, Centers for Disease Control and Prevention Director Dr. Rochelle Walensky signed off on a booster dose for older adults, as well as people at high risk of exposure or severe disease. 

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While a third shot can officially start going into arms, there is a caveat: Only people who have already been fully immunized with Pfizer-BioNTech’s two-dose Covid vaccine are eligible for a booster shot, which must also be Pfizer’s, based on the decisions made this week. 

That leaves out millions of Americans who were initially vaccinated with Covid vaccines made by either Moderna or Johnson & Johnson — a decision that frustrated a number of advisers at a meeting of the CDC’s advisory committee, which met Wednesday and Thursday to discuss the booster recommendations. 

But the Food and Drug Administration stood by its decision to not endorse a “mix-and-match” approach to vaccination boosters —meaning a booster shot of a different vaccine than what was used for the initial vaccination — citing a lack of evidence.

“Consequently, the Pfizer-BioNTech Covid vaccine is authorized for use as a booster dose among certain individuals who completed a primary series of the Pfizer-BioNTech Covid vaccine,” Dr. Doran Fink, a deputy director of the FDA’s Division of Vaccines, said Thursday.

That means those who were initially vaccinated with Moderna’s or Johnson & Johnson’s vaccines will have to wait. 

Surgeon General Vivek Murthy sought to reassure people who had received Moderna’s and Johnson & Johnson’s shots during a White House Covid briefing Friday.

“Your health matters just as much as other vaccine recipients, and we want to make sure that your protection against Covid is strong and reliable as well,” he said. “That’s why the FDA is working closely with Moderna and Johnson & Johnson to get and process their data as quickly as possible, with the goal of making booster recommendations from Moderna and Johnson & Johnson recipients in the coming weeks. This is a high, high priority.”

Moderna submitted an emergency use authorization application for their booster earlier this month.

“The Moderna vaccine is a very similar vaccine to the Pfizer vaccine, so I would anticipate an approval for that could follow shortly,” said Dr. Christina Rostad, an assistant professor of pediatric infectious diseases at the Emory University School of Medicine.

A decision on Moderna’s vaccine will not come until the data is discussed during a meeting of the FDA’s Vaccines and Related Biological Products Advisory Committee. No meeting has been scheduled at this time.

Johnson & Johnson released data from a Phase 3 clinical trial for its booster, which showed positive results, but the company has not applied for emergency use authorization yet. 

Growing evidence

Rostad acknowledged why the FDA did not endorse mixing and matching vaccines as of Friday. 

“There’s very limited data on mixing and matching vaccine types and essentially no data on mixing and matching delayed booster vaccines,” she said.  

But research on mixing and matching is underway.

A National Institutes of Health clinical trial is studying how people respond to receiving a booster  shot that’s a different vaccine than their initial full dose. The trial is currently in the first of two phases.

“There isn’t an immunological reason to think that mixing two mRNA vaccines would be problematic in any way, since the two vaccines are quite similar,” Dr. William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health, said referring to the Pfizer and the Moderna vaccines. “The issue is that there’s just not the data yet on that combination.”

A man receives the Covid vaccine from registered nurse Mary Jo Vetorino inside the OhioHealth mobile clinic, on Aug. 23, 2021, in Columbus, Ohio.Stephen Zenner / SOPA Images via Getty Images file

“What I think is a more interesting combination is the Johnson & Johnson vaccine followed by an mRNA booster,” he said.

That’s because the two vaccines use different technologies to trigger immunity. The Johnson & Johnson vaccine does not use mRNA but rather uses a weakened version of what’s called an adenovirus. This virus can’t make a person sick; instead, it’s modified to enter the body and teach the immune system how to protect itself against the coronavirus. AstraZeneca’s vaccine works in the same manner.

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Both Moderna’s and Pfizer’s two-dose Covid vaccines use genetic material called mRNA, not an adenovirus, to achieve the same effect of training the immune system. 

Research has shown promising results when the two different types of vaccines — adenovirus and mRNA — are administered closer together. One study from the United Kingdom, posted online as a pre-print (meaning it has not been peer-reviewed) found that combining vaccines during the initial vaccination series — using one dose of AstraZeneca’s and one of Pfizer’s — appears to trigger a stronger immune response than two doses of any single vaccine. 

But because that study looked only at the initial vaccination series, experts don’t yet know how this may apply to booster vaccines. 

According to Dr. Steven Carsons, director of the Vaccine Center at NYU Langone Hospital—Long Island, receiving both an adenovirus vaccine and an mRNA vaccine may provide different pieces of information to the immune system’s memory, potentially better equipping it to quickly recognize and fight off the coronavirus. The same could be true for the two mRNA vaccines, which each use slightly different proteins to train the immune system.

The push for mix and match 

Dr. Nirav Shah, director of the Maine Center for Disease Control and Prevention, said during the CDC advisory committee meeting Thursday that not allowing the mix-and-match approach could create logistical challenges and delay getting boosters to high-risk groups. 

Vaccinators may have to make multiple trips to long-term care facilities to provide different boosters as they are approved. This will also overlap with a rush to get first doses to children, for whom vaccine authorization could come within weeks.

“I recognize Dr. Fink [of the FDA] and others note that we don’t have definitive randomized controlled trial data on the efficacy of mixing and matching,” Shah said. Still, he added that medical experts do “have good reason to believe that the mix-and-match strategy will be applicable.”

But until clinical trials can prove that mixing and matching vaccines is both safe and effective, the FDA is unlikely to recommend the practice. 

“This is how science should work,” Carsons said. “If the panel making these recommendations don’t feel the data is robust enough to make a firm recommendation, then they will modify the recommendation. This ensures that the benefit outweighs the risks.” 

As it stands, “folks who can should go ahead and get a Pfizer booster because that’s the data that we have right now,” he said, referring to people who were initially vaccinated with Pfizer’s shot and are covered under the latest recommendations. 

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