COVID Questions
Updated: August 18, 2021 2:23 PM EDT | Originally published: 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:

Recently, there’s been a lot of talk about whether people who got the mRNA shots made by Pfizer-BioNTech and Moderna should get booster doses. The U.S. Food and Drug Administration’s (FDA) advisory panel voted to recommend third doses for Pfizer-BioNTech recipients who are elderly or at high risk of COVID-19 exposure, but stopped short of recommending them for the general population. Moderna has also asked the agency to authorize a booster dose of its shot, but it’s not clear if or when that might happen.

But what about people, like you, who received Johnson & Johnson’s shot?

Health authorities have not officially recommended that Johnson & Johnson recipients receive boosters. But on Sept. 21, Johnson & Johnson said in a statement that a booster dose offers improved protection against the virus.

Based on a study of almost 400,000 people who received the single-dose vaccine, Johnson & Johnson estimated that its shot remained about 80% effective at preventing COVID-19 hospitalizations through mid-July. But among a smaller group of people who received a second shot two months after their first, protection against moderate-to-severe COVID-19 rose to 94% in the U.S., according to the company. (Worldwide, the statement says, a booster dose was 75% effective against moderate-to-severe disease; the company did not explain the discrepancy, but it may be related to which variants were circulating in countries where the shots were tested.) Two doses offered near-total protection against critical disease.

Johnson & Johnson says it has provided those data—which have not yet been published in a peer-reviewed journal—to the FDA, but it’s not clear if or when regulators would recommend a booster. So, getting back to your immediate question: Should you go get a dose of Pfizer-BioNTech or Moderna for additional protection now, as Delta continues to spread?

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, and initial results are expected soon.

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 have tried out such systems.

Safety aside, a second shot might not even be necessary. 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. A recent CDC study found that, through mid-August, the Johnson & Johnson shot was 71% effective at preventing COVID-19-related hospitalizations, compared to 93% for Moderna and 88% for Pfizer-BioNTech.

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. Researchers continue to study the effects of booster doses and mix-and-match vaccination, and more recommendations will likely be coming soon.

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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