UNC Health Talk

Simplifying Boosters: How to Know When to Get One

COVID-19 booster shots are now available to a huge swath of people—a group that may include you. Find out if you’re eligible for a booster (we can help with that, below) and if you are, don’t delay. Boosters provide a continued layer of protection that will be important as we work to end the pandemic and minimize severe disease.

The Food and Drug Administration (FDA) has expanded its emergency use authorization on COVID-19 vaccines to include booster shots of all three vaccines (Pfizer, Moderna and Johnson & Johnson).

“All three vaccines available in the U.S. continue to show high levels of protection against becoming severely ill with COVID-19. However, there are signals that some people vaccinated earlier may have increasing vulnerability to becoming infected with the virus,” says UNC Health infectious diseases specialist David A. Wohl, MD. “A booster dose bumps up immunity that may be weakening.”

Read on to learn more.

If You Received the Pfizer or Moderna Vaccines

If you received the Pfizer or Moderna vaccine, and it’s been at least six months since your initial doses, you are eligible for a booster if you are 18 years or older—that’s all that’s required. You can get the same brand of shot you got the first time or “mix and match” and choose the other.

People who have moderately to severely compromised immune systems (because of cancer treatment, organ transplant, advanced HIV infection and other conditions) should get another dose even earlier. If you are in this group and received Moderna or Pfizer for your first two doses, you’ll want to get a third shot at least one month after your second shot.

“For patients whose immune systems are compromised, the third shot is not so much a booster but is recommended to try to trigger their immune systems to make a stronger response,” Dr. Wohl says.

The Pfizer booster is the same dose as the original shots; the Moderna booster is a half-dose for those without moderately to severely compromised immune systems. For those who are immunocompromised, the Moderna booster dose is the same as the initial two doses.

If You Received the Johnson & Johnson Vaccine

Anyone age 18 or older who received a single dose of the Johnson & Johnson vaccine two or more months ago is eligible for a booster dose of any of the three vaccines.

Experts are encouraging Johnson & Johnson recipients to boost with an mRNA vaccine (Moderna or Pfizer).

A recent study found that recipients of Johnson & Johnson’s single-dose shot who received a Johnson & Johnson booster had antibodies increase by four times. The antibody boost from the other vaccines was much larger: Antibody levels rose 35-fold with a Pfizer booster and 76-fold with a Moderna booster.

In other words, people who received the Johnson & Johnson vaccine produced stronger antibody levels after they got booster shots made by Moderna or Pfizer, compared to boosters from Johnson & Johnson. It is important to note study participants were given a full dose of the Moderna vaccine.

“Antibody levels are one important indicator of vaccine response, and people who receive two different COVID-19 vaccines have been found to generate very strong antibody responses. Studies also show that side effects are not worse,” Dr. Wohl says. “Some people may decide to mix-and-match and the best data we have seen come from boosting with Pfizer or Moderna after initially receiving the Johnson & Johnson vaccine.”

People who have moderately to severely compromised immune systems because of cancer treatment, organ transplant, advanced HIV infection and other conditions who were initially vaccinated with Johnson & Johnson should receive a booster with a Pfizer or Moderna shot at least one month after their initial dose.

Why Get a Booster If You’re Low Risk?

Boosters are especially important in people older than 65, people with chronic health issues and those at high risk of COVID-19 exposure. But even if you’re fully vaccinated, young and healthy, with no underlying conditions, a booster is a good idea, experts say.

Getting boosted will minimize your chances of getting a so-called “breakthrough” case of COVID-19, which tend to be more mild but can still cause missed work and school. The more people who get booster shots, the less opportunity the virus has to continue to circulate and mutate.

Fully vaccinated people who receive a booster can feel comfortable doing many of the things they enjoyed pre-pandemic, but it’s still wise to avoid indoor crowds and wear a mask when physical distancing isn’t possible.

This article originally ran October 26, 2021 and was updated Nov. 22, 2021.


Visit unchealthcare.org/vaccine for the latest information on the COVID-19 vaccines.