COVID Questions
July 8, 2021 11:59 AM EDT

Welcome to COVID Questions, TIME’s advice column. We’re trying to make living through the pandemic a little easier, with expert-backed answers to your toughest coronavirus-related dilemmas. While we can’t and don’t offer medical advice—those questions should go to your doctor—we hope this column will help you sort through this stressful and confusing time. Got a question? Write to us at covidquestions@time.com.

Today, N.C. in Washington, D.C., asks:

It’s true that vaccines made with mRNA—those from Pfizer-BioNTech and Moderna—offer stronger protection against initial COVID-19 strains than Janssen/Johnson & Johnson’s adenovirus-based shot. That’s why some people who got Johnson & Johnson’s vaccine are interested in doubling up, particularly as the more transmissible Delta variant continues to spread in the U.S.

The U.S. Centers for Disease Control and Prevention’s (CDC) official stance is that COVID-19 shots are not interchangeable. That doesn’t necessarily mean it’s unsafe to mix them; there’re just not a lot of data available to answer the question.

Some prominent researchers have decided to forge ahead and get mRNA boosters on top of Johnson & Johnson shots, even without an official blessing from the CDC—and despite the fact that some pharmacies won’t give additional shots to people who are already fully vaccinated.

Dr. Hana Mohammed El Sahly, a molecular virology and microbiology expert at Baylor College of Medicine, is not one of those researchers—and she says she wouldn’t recommend that course of action. “Until we have better data, it probably is not wise to go and get [additional] vaccines unless it’s part of a clinical trial,” she says.

Those data are on the way. National Institutes of Health (NIH) scientists are studying what happens when people get a booster of a different vaccine than their original shot. That study includes the Johnson & Johnson vaccine.

European research teams have also published results suggesting that it’s effective to mix one dose of Pfizer-BioNTech’s vaccine with one dose of the shot made by AstraZeneca-Oxford University—which, like Johnson & Johnson’s vaccine, is adenovirus-based. That’s not a direct parallel, but it does support the general concept of mixing and matching doses. Countries including Germany, Canada and the Dominican Republic are already trying out such systems, but none of them are mixing Johnson & Johnson shots with other vaccines.

Safety aside, a second shot might not even be necessary. A new study published in Nature found that people who were only partially vaccinated (i.e., who received only one shot out of a two-dose regimen) were susceptible to the Delta variant, but that fully vaccinated individuals were much better protected. The study didn’t analyze Johnson & Johnson’s shot, but people who get the one-dose vaccine are considered fully vaccinated by the CDC.

Johnson & Johnson says its vaccine—like others authorized in the U.S.—works well against the Delta variant, although that conclusion came from a very small study. Even with the Delta variant spreading, El Sahly notes, the vast majority of people who end up hospitalized or die from COVID-19 are unvaccinated, which suggests the shots continue to work well as authorized.

If you’re concerned, El Sahly says your best bet, at least for now, is returning to precautions like masking and social distancing—not getting a second type of vaccine.

That advice may change down the road. Experts have said it’s possible that everyone will need a booster shot roughly a year after vaccination, depending on how long they provide protection against the virus. By the time health officials know whether boosters are necessary, El Sahly says, researchers will hopefully have gathered enough data to make a recommendation about whether you should stick with Johnson & Johnson or switch to Pfizer-BioNTech or Moderna.

And if you’re absolutely desperate to get double-vaccinated in the meantime, the NIH’s study on mix-and-match dosing is enrolling now. “You can volunteer there and help us generate those data,” El Sahly says.


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Write to Jamie Ducharme at jamie.ducharme@time.com.

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