What to know about booster shots and third doses of the COVID-19 vaccine

Clinician provides a vaccine to a patient

Increasingly, research shows many people who have weakened immune systems haven’t been able to receive full protection from their COVID-19 vaccines. Unlike healthy people who’ve been vaccinated against SARS-CoV-2, the virus that causes COVID-19, immunocompromised people’s bodies may not produce enough protective antibodies after two doses of the COVID-19 vaccines. This could leave them particularly vulnerable to COVID-19, especially as more contagious variants circulate in a community.

That’s why federal health authorities announced in mid-August that they would allow immunocompromised people to get special, additional doses of the Moderna and Pfizer /BioNTech vaccines and said later they'd begin booster shots for healthy, immunized people later this fall.

As an infectious diseases specialist and hospital epidemiologist at the University of Chicago Medicine, I think it’s critical that people understand why this extra dose is needed and why not everyone needs one just yet. Here’s what you need to know about the third dose of the COVID-19 vaccine.

Am I eligible for a third dose of the COVID-19 vaccine?

In the United States, people with weakened immune systems are eligible for a third dose of the COVID-19 vaccine along with people who are 65 and older and those who live or work in certain high-risk settings. While the criteria will most likely change, this group currently includes patients who’ve had or are receiving:

  • Organ transplants
  • Stem cell transplants within the past two years
  • Active cancer treatment for tumors or blood cancer and are undergoing chemotherapy that affects the immune system
  • Severe primary immunodeficiency
  • Advanced or untreated HIV
  • Active treatment with high-dose corticosteroids or other drugs that may suppress your immune response.

There is not enough data yet to know whether immunocompromised people who got the one-dose Johnson & Johnson vaccine need another dose, but we expect to know more soon.

You are not eligible for a third dose of the vaccine at this time if you are healthy, do not have one of these specific conditions, do not take certain immunosuppressive medications, are under 65, or do not live and work in a high-risk setting.

Is a third dose the same thing as a booster dose?

No. A booster is given to people who got a full course of a vaccine and developed a good response. For some vaccines, antibodies and other aspects of a person’s initially strong immune response start to decrease (or wane) over time. When that happens, people are offered booster doses to pump their immune response back to previous levels.

Unlike boosters, third/additional doses of COVID-19 vaccines are for people who received the complete starter series of vaccines but then their immune systems didn’t have a good enough response. Evidence shows these are generally people whose immune systems are weaker. That’s why the FDA and CDC are recommending an additional dose for immunocompromised individuals.

Scientists are watching COVID-19 vaccine recipients closely to see if there is evidence of waning immunity and some countries have even begun offering booster doses to older people or those who were vaccinated in early phases of vaccine clinical trials. In the U.S., federal health officials suggest healthy people who got the Moderna or Pfizer vaccines receive a booster eight months after their second dose. 

Does my booster vaccine or third dose need to be the same brand as my initial vaccine?

If possible, yes. The CDC recommends that booster doses match the original mRNA vaccines people received earlier this year. This subsequent dose should be given at least 28 days after the second dose of the vaccine. If you absolutely cannot find a matching dose of vaccine, it would be OK to get the other one.

If I am immunocompromised, will I be fully protected after I get my third dose of the vaccine?

No. If you are immunocompromised, a third dose is supposed to provide you with better protection from COVID-19, but it may not provide you with the same level of immunity as healthy people. In studies, most participants who had any immune response to the first two doses did better after a third dose, but some people didn’t. That means you should still take extra precautions to avoid getting COVID-19, such as wearing a mask (especially inside) and avoiding large crowds or high-risk activities. Consider wearing eye protection (like a face shield), particularly if you’re around unmasked people who may be unvaccinated.

Discuss your ongoing risk with your healthcare provider and ask them how you should reduce your risk. It’s important that your family members and regular close contacts get vaccinated, too. This decreases the likelihood that you will be exposed to COVID-19.

I have a health condition that isn’t on the CDC’s third-dose list. Can I still get my COVID-19 booster shot?

If your health condition is not on the CDC’s third-dose list, then you are not yet eligible for a third shot within the immunocompromised category. If you aren’t sure whether your condition counts, contact your doctor. Hospitals, doctors’ offices and pharmacies can give third doses only to people who federal public health authorities have said should receive the additional doses. Other people will be able to get booster shots as early as this fall. Federal health officials say people shouldn't get boosters until eight months after their second dose.

Why didn’t we know third doses would be needed before?

We’re still learning about the COVID-19 vaccines and how well — and for how long — they protect us from the virus and its emerging variants. The CDC’s decision to provide third doses to immunocompromised people was made after data showed these groups were particularly vulnerable to infections since they had little protection from their earlier immunizations. We will keep learning about these vaccines in the coming months and years, which means we will adjust our vaccine plan accordingly.

Will more people keep needing additional doses of the COVID-19 vaccines?

It’s almost certain that most people will eventually need to get additional doses of the vaccines. However, we are still studying how long the vaccines will protect people. The good news is that people who are healthy should still have ample protection from their COVID-19 vaccines. While some healthy, vaccinated people do get COVID-19 infections, they are much less common than in unvaccinated people and they are much less severe. This is why it’s important for people who haven’t received their vaccine yet to get it right away.

COVID-19 vaccination record card with vaccine viles and needle

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Emily Landon, MD

Emily Landon, MD

Dr. Emily Landon specializes in infectious disease, and serves as Executive Medical Director for infection prevention and control.

Learn more about Dr. Landon.