December 19, 2021

Here’s Why You Need a Third Dose of the COVID-19 Vaccine

An expert explains the science behind the decision

A metal tray with vials of the COVID-19 vaccine, bandages and needles

With the omicron variant driving yet another new surge of COVID-19 cases, including a rising number of breakthrough cases for fully vaccinated people, getting your third dose of the COVID-19 vaccine is more critical than ever.


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Data suggests that while the current vaccines have so far proven effective against COVID-19, the protection they offer may fade after a while. Using that data, the U.S. Department of Health and Human Services (HHS), the CDC, the FDA and other experts in the medical field and federal government starting distributing a third dose of the COVID-19 vaccine in the fall of 2021. In a joint statement, the group said, “We conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability.”

By October 2021, the FDA expanded the list of those eligible for an additional dose of any of the three COVID-19 vaccines. To better understand the need for a third dose of the vaccine, we talked to digestive disease expert and Lerner Research Institute Chair, Thaddeus Stappenbeck, MD, PhD.

Why do I need a third dose?

New research indicates that the protection the two mRNA vaccines – Pfizer and Moderna – create against COVID-19 might fade after several months. The data comes from a study of vaccinated patients in Israel, where the Pfizer vaccine was the predominant option and vaccinations began in December 2020, ahead of many other countries, including the United States.

According to the study, as the delta variant spread through Israel in the early summer of 2021, there was a correlation between receiving the vaccine at an earlier date and contracting a breakthrough case. For example, patients vaccinated in January 2021 were 2.26 times more likely to contract a breakthrough infection than those vaccinated in April 2021.

“The trials for the mRNA vaccines showed us they were up to 94% effective in protecting patients from infection,” says Dr. Stappenbeck. “But because of the situation, the other main point of those trials was to establish safety so we could start vaccinating people. We’re still learning how long that protection lasts and based on what we’ve seen in Israel, it starts to fade around six months for some people.”


He adds that the data hasn’t been investigated deeply enough yet to see if there’s evidence of why the immunity level of some people may drop. “What’s clear, though, is that more people are being exposed to the delta variant, and more people that are vaccinated are getting breakthrough infections than the initial data suggested,” he says.

How do I know if my immunity is fading?

Unfortunately, there’s no easy way to measure if your immunity to COVID-19 is fading, particularly if you’re a healthy adult, says Dr. Stappenbeck. “Immunity is a multi-factor thing that’s not just about the number of antibodies to a specific virus that you have in your bloodstream,” he explains. “There’s a cellular-based immunity, too, and that’s harder to measure.”

The biggest challenge for experts is that COVID-19 continues to evolve, mutate and force doctors and researchers to adapt on the fly. “It’s a moving target,” Dr. Stappenbeck says. “New data is coming out every day.”

It’s important to note that the Pfizer and Moderna vaccines are incredibly effective against the delta variant and seem to dramatically lessen the likelihood of severe illness from the omicron variant. Pfizer shared data with the FDA on this point, saying, “The data we’ve seen to date suggests a third dose of the vaccine elicits antibody levels that significantly exceed those seen after the two-dose primary schedule.”

Who is eligible for the third dose?

In the U.S., everyone ages 16 and older can — and, doctors say, should — get a booster shot. The CDC’s guidelines are as follows:

  • If you originally received the Pfizer vaccine: You should get your booster at least six months after completing your primary vaccination series. Teens ages 16 and 17 can get a Pfizer vaccine booster, while adults 18 years and older can get a booster of any of the COVID-19 vaccines authorized in the U.S.
  • If you originally received the Moderna vaccine: You should get your booster at least six months after completing your primary COVID-19 vaccination series, and you can can get any of the COVID-19 vaccines authorized in the U.S. This vaccine is not available to teens ages 16 and 17.
  • If you originally received the Johnson & Johnson vaccine: You should get your booster at least two months after completing your primary COVID-19 vaccination series, and you can can get any of the COVID-19 vaccines authorized in the U.S. This vaccine is not available to teens ages 16 and 17.

What if I got my original vaccine abroad?

If you were originally vaccinated outside the U.S., you’re eligible for a Pfizer-BioNTech booster at least six months after your original vaccines, so long as you are 18 or older and have done one of the following:

  • Received all the recommended doses of a World Health Organization emergency use listing COVID-19 vaccine not approved or authorized by the FDA.
  • Completed a mix-and-match series of any combination of FDA-approved, FDA-authorized, or WHO-EUL COVID-19 vaccines.

When will children be eligible for boosters?

Children ages 5 to 11 can receive the Pfizer COVID-19 vaccine, but doctors don’t yet know when or if third doses will be recommended for children under 16.

What should I do while I wait for my third dose?

If you’re not yet eligible for your third dose, the best thing to do, Dr. Stappenbeck says, is to continue practicing social distancing guidelines, including wearing masks, maintaining six feet of distance and keeping up with good hand hygiene.

In the meantime, he says, continue to be safe, and if you’re not already, get vaccinated now. “All of the real-world data suggests the vaccines are effective against variants and that they’re the best protection you have,” he says.

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