In absence of federal guidance, some people—including immunocompromised patients, as well as those who have received only a single-dose Covid-19 vaccine—are weighing whether they should, or even can, get an mRNA booster shot.

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Why are some people considering booster shots?

According to the Washington Post, some patients who have received the single-dose Johnson & Johnson (J&J) vaccine are concerned about the relative paucity of evidence on the vaccine's efficacy in comparison to the two authorized mRNA vaccines, from Pfizer-BioNTech and Moderna.

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As Michelle Goldberg writes for the New York Times, "when [CDC} said that vaccinated people don't have to wear masks indoors, it was primarily based on findings that the mRNA vaccines … inhibit transmission"—not the J&J vaccine. And when a British study found that the protection afforded by a single dose of Pfizer-BioNTech's vaccine or AstraZeneca's vaccine—which, like J&J's shot, is a viral vector vaccine—declined substantially against the more contagious delta variant, to about 35%, no one knew for " weeks" what that might mean for the J&J vaccine's efficacy, according to the Times.

Similarly, some people who have completed their vaccine course with any of the three authorized vaccines and who are also immunocompromised are weighing whether to get a booster shot to improve their overall antibody levels.

According to the Times, immunocompromised people often have a lower antibody response to vaccines, which is why they tend to get booster shots of other vaccines, such as for the flu or for hepatitis B. For instance, recent research suggests that less than 50% of people who had an organ transplant had detectable antibody levels after receiving two doses of either authorized mRNA vaccine—and a follow-up study indicated that among 30 organ transplant recipients who had low or undetectable antibody levels following two mRNA doses, a third dose appeared to improve antibody levels.  

What is the current guidance?

According to the Washington Post, neither CDC nor FDA has yet issued firm guidance on whether people who have received Johnson & Johnson's single-dose Covid-19 vaccine should get a booster shot of either authorized mRNA vaccine.

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FDA said, "There are no data available on the interchangeability of the [J&J] vaccine with other COVID-19 vaccines … The FDA would need to review any new data regarding the use of a different vaccine booster from the original vaccine an individual received to make a determination about what kind of authorization would be needed."

Separately, CDC Director Rochelle Walensky last week said the agency has "no information to suggest that you need a second shot after J&J, even with the delta variant."

Similarly, federal health agencies have not issued any guidelines on whether people who are immunocompromised should receive a booster shot, the New York Times reports. In fact, according to the Times, CDC, FDA, and NIH currently recommend against antibody testing after Covid-19 vaccination, which makes it difficult for providers to determine who may even need additional protection.

Are booster shots safe?

Although research on Covid-19 booster shots is limited, the available evidence—and a certain amount of precedent—suggests that such shots are likely safe and potentially effective at improving overall protection, the New York Times reports.

For example, early research suggests that mixing different vaccine doses is safe, although the practice may increase the severity of side-effects, the Times reports. For instance, a British study found no safety issues among patients who had received one dose of AstraZeneca's vaccine and one mRNA dose. And a Spanish study found that following an AstraZeneca dose with a mRNA dose produced stronger levels of protection than did two AstraZeneca doses.

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Similarly, multiple studies suggest that a third Covid-19 vaccine dose may improve antibody levels in patients who did not present any detectable antibody levels after a first or second dose of a Covid-19 vaccine. And both Moderna and Pfizer are planning studies to test the efficacy of a third vaccine dose among organ transplant and other immunocompromised patients, the Times reports.

All that said, experts cautioned that much still remains unknown about booster shots. For instance, Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, said while adding a booster shot to any of the authorized vaccines will likely increase immune response, "how that translates into clinical protection, that's the part that's not yet known."

"People want the science to tell us exactly what to do," Andy Slavitt, who until recently served as a senior adviser to President Joe Biden's pandemic response team, said. "When we say, 'Follow the science,' the reality is the science doesn't always know."

Moreover, currently available research suggests that there's no need for a booster shot in most people, regardless of which vaccine course they originally completed. In addition to the growing body of evidence indicating that Pfizer-BioNTech and Moderna vaccines continue to protect against all known variants, new research—although not yet peer reviewed—also suggests that J&J's vaccine provides "robust neutralizing antibody responses and good cross-reactivity against all the variants, including delta," Barouch said.

Robert Atmar, a professor of infectious diseases at Baylor College of Medicine, recommended against getting a booster shot in absence of further evidence about risk and benefits, the Post reports. Atmar said, "From a public health standpoint, what we need to do, in my opinion, is try and get our populations who aren't vaccinated to accept a vaccine that's readily available, and that will have a much larger impact than trying to roll out a booster strategy at this point." (Guarino/Chiu, Washington Post, 7/4; Mandavilli, New York Times, 7/4; Goldberg, New York Times, 7/2)