Your Questions About COVID-19 Booster Shots, Answered

An FDA panel recommended boosters for all J&J recipients, as well as for people 65 and up and in certain high-risk groups who got Moderna or Pfizer.

A healthcare worker prepares Pfizer-BioNTech Covid-19 vaccines for third doses at a senior living facility in Pennsylvania.
A healthcare worker prepares Pfizer-BioNTech COVID-19 vaccines for third doses at a senior living facility in Pennsylvania.
Photo: Hannah Beier/Getty Images

In August, U.S. health officials announced plans to distribute a third “booster” shot to Americans who had received either the Pfizer or Moderna coronavirus vaccines, starting as soon as Sept. 20. The plans were always contingent on a data review and recommendations from the Food and Drug Administration and the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices (ACIP).

Last month, these agencies recommended boosters for anyone 65 and up who received the Pfizer-BioNTech vaccine at least six months ago, for people 18 to 64 with health conditions that put them at high risk for severe COVID-19, and for people 18 to 64 whose jobs or living situations put them at high risk for COVID-19. Yesterday, an advisory panel to the FDA voted to recommend boosters for people in these same populations who received the Moderna vaccine at least six months ago.

And on Oct. 15, that FDA panel recommended that anyone who received a Johnson & Johnson vaccine at least two months ago get a booster shot as well. One question that has not yet been answered is whether the FDA will eventually authorize J&J shot recipients to get a booster with an mRNA vaccine (Pfizer or Moderna), which some data indicates might provide stronger protection.

For Moderna and J&J shot recipients waiting for a booster, a few steps still remain. The FDA is expected to sign off on the panel’s recommendations shortly. Then, later next week, the CDC’s ACIP—which sets recommendations for vaccines in the U.S.—will vote. After that vote, CDC Director Rochelle Walensky will issue official recommendations from the CDC, which is when boosters for Moderna and J&J recipients could actually become available.

Booster shots are already available for people in eligible populations who got the Pfizer shot.

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Since the August announcement, there’s been significant discussion among health experts about whether or not it’s too soon to recommend a booster dose for the general public, especially for people who received an mRNA vaccine. Pfizer-BioNTech and Moderna have released data indicating that the protection offered by those vaccines against infection has waned over time. But the data also shows that those vaccines still appear to offer excellent protection against severe disease, hospitalization, and death. 

To help cut through the confusion, CR talked to experts and examined the data to better understand what we know about boosters, who might need them, and why they have sparked debate in the scientific community.

What Are the Current U.S. Recommendations About Boosters?

A third shot was first recommended only for people who are seriously immunocompromised, beginning in August. This includes those who are on immunosuppressant drugs after an organ transplant as well as people who have a similar level of immune system impairment, one which has prevented their immune systems from generating sufficient protection after the first two shots. Research indicates that for people who have received an organ transplant, a third dose can safely and significantly raise levels of protective antibodies.

For people who got the Pfizer-BioNTech vaccine at least six months ago, the FDA authorized and the CDC recommended boosters for people 65 and older and for people 50 to 64 with medical conditions that put them at greater risk for severe COVID-19. The CDC also recommended that people 18 to 49 at risk for severe COVID-19 who were immunized with the Pfizer-BioNTech vaccine at least six months ago be offered a booster shot, based on an individual analysis of the risks and benefits of the booster for them, or based on a discussion with their healthcare provider. People vaccinated with Pfizer-BioNTech who have a high risk of occupational or institutional exposure—including those like healthcare workers and those who work in settings where they could frequently be exposed, like in schools—may also be offered a booster shot, the agency said. These booster shots are already available.

Recommendations for people who got the Moderna and J&J shot should be announced within a week or so, now that an FDA advisory panel has recommended boosters for many of these shot recipients.

What’s the Evidence That People Need Boosters?

Pfizer-BioNTech, Moderna, and Johnson & Johnson have released data to help explain why they believe a booster shot would be beneficial.

In data that Pfizer and BioNTech submitted to the FDA, the companies cited a preliminary study by healthcare company Kaiser Permanente, which found that the vaccine’s efficacy against infections with the Delta variant declined from 93 percent a month after initial vaccination to 53 percent four or more months after vaccination. Efficacy against hospitalization remained at 93 percent. Evidence presented by Pfizer indicated particular concerns about immunity waning for older people.

On Sept. 15, Moderna released data showing that people who were immunized with that vaccine between December 2020 and March 2021 were about 36 percent less likely to develop a breakthrough infection than people who were immunized between July and October 2020. The company says that this apparent decline in protection from infection supports the use of a booster dose. However, the company said that the vaccine was still about 95 percent effective at preventing hospitalization with COVID-19. 

In its submission to the FDA requesting authorization for a booster dose, Johnson & Johnson noted that while its vaccine remained highly effective against the most severe cases of COVID-19 (which resulted in hospitalization or death), its efficacy against moderate to severe COVID-19—which was lower than the mRNA vaccines from the start—had fallen over time, driven by reduced protection against variants of the coronavirus. During the FDA advisory committee meeting discussing J&J boosters, FDA and CDC experts stressed that the lower level of protection for people who had only received one J&J shot made a booster particularly important for this group.

It’s not entirely clear how much the decline in protection shown by studies like these is due to immunity waning over time and how much is caused by the increased transmissibility of the Delta variant, says Gregory Poland, MD, director of the Vaccine Research Group at the Mayo Clinic in Rochester, Minn. According to the researchers behind the Kaiser analysis, it’s likely the decline is primarily due to waning immunity, though more research on this is needed.

During the August announcement on boosters, health officials said the apparent decline in protection against any infection—defined as testing positive for the coronavirus, with or without symptoms—could be followed by a decline in protection against severe disease.

But that decline in protection against severe disease has not yet been observed in the U.S. Though Israel has reported that older adults are experiencing declining protection against severe disease, preliminary data from the U.K. indicates that protection against severe illness there remains strong. The FDA advisory panel, however, determined that the evidence that protection against infection was waning was strong enough to warrant recommending booster shots for certain high-risk groups.

Still, for many experts, it’s too soon to say that younger people who are not high-risk need to get a booster shot—though that could change over time. “There remains no overwhelmingly convincing data for boosters for the general population at this stage since most people received their vaccination less than a year ago,” says Joshua Barocas, MD, an associate professor of medicine at the University of Colorado School of Medicine, who specializes in infectious disease. “This might change as the virus continues to evolve.”

Do Boosters Improve Immunity?

According to a study published October 7 in the New England Journal of Medicine, a booster shot for people in Israel 60 and older who had been fully vaccinated at least five months earlier dramatically reduced the risk for any breakthrough infection or for severe disease.

Data submitted by Pfizer to the FDA also indicates that a booster shot dramatically raises antibody levels, a partial measure of the body’s immune response. Data from Moderna shows a significant boost to antibody levels with a Moderna booster, and data from Johnson & Johnson shows the same for a J&J booster.

But while it’s no surprise that a booster raises antibody levels, that doesn’t mean everyone needs that shot yet, says Poland. That’s because the antibody levels generated by vaccination in the first place still appear to be high enough to protect the average person. 

There may be certain people who do need that additional dose, he says. A third shot has already been recommended for immunocompromised people because research has found they do not generate nearly as many antibodies as others in response to the first two shots. For that reason, experts say the third shot for these groups should ​​not be considered a “booster,” but just part of their initial vaccination. Certain elderly and frail people or people with certain constellations of risk factors, like having diabetes and heart disease, might be the next to need a booster, Poland says.

Are There Additional Risks to a Booster Shot?

The initial safety data on a third vaccine dose looks promising, with no immediate red flags, according to Anna Durbin, MD, director of the Center for Immunization Research and a professor in the department of international health at Johns Hopkins Bloomberg School of Public Health. That said, experts say they still want to see more safety data before boosters are advised for everyone. 

Even if safety data looks good so far, we don’t yet know if a booster shot could increase the risk for myocarditis (heart inflammation) or other rare side effects for certain populations, says Poland. 

The risk of rare side effects is one reason why Poland wouldn’t recommend rushing to get an additional dose before experts have a chance to fully weigh the risks and benefits.

Should I Get the Same Shot I Got the First Time?

For now, yes, especially if you got a Pfizer or Moderna shot first. For people who do need a third dose, the CDC recommends that they get the same mRNA vaccine they got the first time around. If that same vaccine is unavailable, or if the person does not know which vaccine they received and has lost their record, either mRNA vaccine is acceptable for a third dose.

However, Moderna’s booster shot is a different dose than the first shot—50 micrograms instead of 100. When recommendations for a Moderna booster are issued, you’ll want to be clear you are getting a booster and not an initial shot. Pfizer’s 15 microgram dose was unchanged.

FDA’s advisory committee panel has only voted to recommend a J&J booster for J&J shot recipients. But a study conducted by the NIH indicates that a Moderna or Pfizer booster may raise antibody levels for J&J recipients far more than a J&J booster—with a Moderna booster providing the biggest jump in antibody levels. Still, this research is preliminary; health officials have not yet made any decisions about using so-called mix-and-match boosters. It’s possible that FDA may authorize a booster with a different shot for J&J shot recipients in the future, according to Peter Marks, the director of the Center for Biologics Evaluation and Research at the FDA, speaking for the agency at the J&J advisory committee discussion.

For now, only people who received a Pfizer shot should get a booster, according to current recommendations. But health officials will vote and craft recommendations on boosters for people who got Moderna and J&J shots over the next week.

What If I Got J&J the First Time?

On Oct. 15, the FDA advisory committee recommended that J&J shot recipients get a booster with the same shot at least two months after their initial dose. After FDA and CDC authorize these boosters and craft recommendations for them, booster shots should become available.

According to a report released by the CDC on Sept. 17 that looked at hospitalizations at 21 U.S. hospitals in 18 states between March 11 and August 15, J&J efficacy against hospitalization was the lowest of the three vaccines initially authorized for use in the U.S., with a 71 percent efficacy against hospitalization. (Moderna efficacy against hospitalization was 93 percent, and Pfizer-BioNTech was 88 percent, according to that report.)

On Sept. 21, J&J released data that showed that a second dose of its shot two months after the first provided 94 percent protection against COVID-19 in the U.S. during the study period, and that a booster shot six months after the initial dose significantly boosted infection-fighting antibodies.

The interesting question that remains is if or when whether health officials will recommend that J&J recipients get a mRNA booster, based on the NIH’s studies of how these boosters may improve protection for people whose first shot was J&J.

Are the Booster Formulas Being Changed to Match Emerging Variants?

The booster vaccine doses that are being given so far are the same formula as the initial vaccine doses. We don’t need a vaccine that specifically targets the Delta variant of the coronavirus because current vaccines still provide excellent protection against severe disease caused by that variant, according to Durbin.

However, companies are working on versions of their vaccines that can target new variants if that is needed, Poland says. Modifying the mRNA vaccines to target a different strain is a relatively quick process, he says. What’s unclear at this point, he says, is how the FDA will regulate these modified vaccines—whether they would treat them as new products (which could take a long time for approval or authorization) or more like annual flu vaccines (which would be much faster).

Will We Just Need Boosters Forever?

Down the road, it’s possible that we may need a regular booster against the coronavirus like we do against the flu, according to Barocas, of the University of Colorado.

But whether or not that’s the case depends on how well we control the ongoing pandemic, according to Durbin. The best way to ensure that we don’t need them soon or don’t need to get them regularly is to stop the disease from spreading—and the best way to do that is to vaccinate the unvaccinated, both in the U.S. and around the world. The more unvaccinated people there are, the more chances there are for new variants to emerge.

Are There Other Reasons to Hold Off on Boosters?

On Sept. 13, a group of vaccine researchers—including two senior officials who recently announced plans to resign from their positions at the FDA’s Office of Vaccines Research and Review—published an article in the medical journal The Lancet saying that for now, though protection against any infection appears to be waning, vaccine protection against severe disease and hospitalization appears to be holding strong. Because of that, the authors wrote, the limited available supply of vaccines should be used primarily to vaccinate the unvaccinated around the world. 

“Even if some gain can ultimately be obtained from boosting, it will not outweigh the benefits of providing initial protection to the unvaccinated,” they wrote.

For that same reason, the World Health Organization has asked wealthy countries to hold off on distributing booster shots until at least the end of this year.


Head shot image of CRO Health editor Kevin Loria

Kevin Loria

I'm a science journalist who writes about health for Consumer Reports. I'm interested in finding the ways that people can transform their health for the better and in calling out the systems, companies, and policies that expose patients to unnecessary harm. As a dad, I spend most of my free time trying to keep up with a toddler, but I also enjoy exploring the outdoors whenever possible. Follow me on Twitter (@kevloria).