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COVID-19 Vaccine Third Dose vs. COVID-19 Booster Shots

It takes at least 2 weeks to develop full immune protection after getting your final dose of COVID-19 vaccine.

You are considered fully vaccinated if it has been at least:

  • 2 weeks since your second dose of the Pfizer-BioNTech or Moderna vaccine
  • 2 weeks after 1 dose of the Johnson & Johnson vaccine

Because protection after vaccination may decrease over time, you may also need a COVID-19 booster shot after some period of time.

If you are immunocompromised (have a weak immune system), your doctor may recommend an additional dose of COVID-19 vaccine as part of your initial vaccination series.

People who are immunocompromised may need an additional vaccine does to help protect against COVID-19.

The third dose of a COVID-19 vaccine helps those with weak immune systems who may not have built up enough immunity from the first two doses of Pfizer-BioNTech or Moderna.

The booster shot helps combat a decrease in immune response over time.

COVID-19 Vaccine Third Dose

Studies have shown that people with weakened immune systems may have a reduced immune response to the vaccine. This results in less protection against severe COVID-19.

People who may need an additional dose of COVID-19 vaccine include patients who:

  • Are currently on active treatment for cancer
  • Have had CAR T-cell therapy or stem cell (bone marrow) transplant within the last 2 years
  • Have an immune system disorder including advanced or untreated HIV infection
  • Have had an organ transplant and are taking medicines that suppress the immune system to prevent rejection of the organ
  • Are taking medicines that severely suppress the immune system such as high-dose corticosteroids, chemotherapy, or other biologic drugs

The third dose can help boost your immune system’s response to the vaccine. You should get your third dose at least 28 days after your last dose. It should be the same vaccine you received previously. Currently third doses are authorized for those age 12 and older who were vaccinated with Pfizer-BioNTech and those aged 18 and older for those who were vaccinated with Moderna.

If you have questions about whether you’re eligible for a third dose, talk with your doctor.

COVID-19 Booster Shots

Studies have shown that immunity may decrease over time.

The Food and Drug Administration (FDA) authorized boosters for all adults. And the Centers for Disease Control and Prevention (CDC) has updated guidance on eligibility for boosters.

  • People over age 18 who received a Pfizer-BioNTech vaccine should receive a booster dose at least 6 months after completing their second dose.
  • People over age 18 who received a Moderna vaccine should receive a booster dose at least 6 months after completing their second dose. The Moderna booster dose is half of the dose that is used in the primary series.

Those 18 and older who received a single dose of Johnson & Johnson should receive a single Johnson & Johnson booster dose at least 2 months after completing their first dose.

In general, a COVID-19 vaccine booster dose should be the same vaccine brand as what you originally received. However, you may choose to get a different COVID-19 vaccine as a booster shot. Mix and match dosing for booster shots is allowed under current recommendations. Talk to your health care provider if you have questions.

How antibodies work after receiving extra doses and booster shots.


Reviewed: November 2021