UNC Health Talk

Simplifying Boosters: How to Know When to Get One

Editor’s note: This article originally ran October 26, 2021, and was updated November 22, 2021, and January 10, 2022 to reflect updated public health guidance.


COVID-19 booster shots have become incredibly important as we face another surge of the coronavirus, with case counts at their highest ever. People who are vaccinated and boosted have the most protection against infection and illness from the omicron variant.

“A booster dose bumps up immunity that may be weakening,” says UNC Health infectious diseases specialist David A. Wohl, MD.

Read on to learn if you’re eligible for a booster, and if you are, don’t delay.

If You Received the Pfizer or Moderna Vaccines

If you received the Pfizer or Moderna vaccine, and it’s been at least five months since your initial doses, you are eligible for a Pfizer booster if you are 12 years or older and a Moderna 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.

From there, immunocompromised people can follow the booster schedule recommended by the Centers for Disease Control and Prevention.

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 shot.

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 two months 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.

Today, with cases skyrocketing past any previous point in the pandemic, we’re all at high risk of exposure. Boosters are an important tool to protect individuals and the community from further infection and illness. Getting boosted reduces your chances of getting infected, and if you do get infected, the booster likely will prevent any serious illness.

This article originally ran October 26, 2021 was updated Nov. 22, 2021 and  Jan. 10, 2022.


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