Who Is Eligible to Receive a Third Dose of COVID-19 Vaccine Right Now?

Sep. 27, 2021 - Katie McCallum

Since you get a flu shot every single year, you might be wondering if the same will be true for the COVID-19 vaccine — especially as more and more variants arise and spread around the world.

Cue the discussion about third shots and boosters.

To cut through the confusion, Dr. H. Dirk Sostman, chief academic officer of Houston Methodist, is here to answer your questions regarding what we know about third shots and boosters right now, as well as what we don't.

Q: What’s the difference between a third shot and a booster?

Dr. Sostman: The term “third shot” is used to describe a third dose that’s needed when a person’s immune response likely hasn’t responded fully to the vaccine after the first doses — such is the case for  people who have undergone solid organ transplants and others who are seriously immunocompromised.

Given this, the FDA expanded the emergency use authorization (EUA) of the Pfizer and Moderna COVID-19 vaccines on Aug. 12 to include third shots for those with weakened immune systems. These people are eligible to receive a third dose without a prescription, given at least 28 days after the second vaccine shot. For now, the third shot is simply another dose of whichever vaccine (Pfizer or Moderna) they initially received.

A booster is also a third dose, but it’s used when help is needed to prolong protective immunity in someone who responded fully after the first two doses but there’s evidence that protection is waning after some time. In essence, a "top-up" of a person's effective immune response to the first vaccine series.

On Sept. 22, the FDA expanded the EUA of the Pfizer COVID-19 vaccine to include booster shots for people who are: over the age of 65 or over the age of 18 and high risk for developing severe COVID-19 or at high occupational risk for COVID-19. These people are now eligible to receive a third dose without a prescription starting six months after their second dose of vaccine. A third dose of Moderna as a booster is not yet available, but we expect a similar authorization before too long.

As the highly infectious Delta variant continues to spread, experts believe third shots and boosters will increase antibody protection in these people who are most vulnerable.

The agency voted against extending Pfizer’s EUA to include boosters for the general public for the time being — squashing the Biden administration’s hope to have boosters available to everyone starting earlier this week.

Q: Why did the FDA vote against making boosters available for everyone?

Dr. Sostman: For most people, the vaccines still provide excellent protection against severe illness and death. This was what the FDA panel focused on. They felt that the main job of the vaccines is to prevent hospitalization and death and that, in most people, the initial doses of  COVID vaccines are still doing this.

While early data is showing that protection against mild and moderate COVID-19 via the Pfizer vaccine may be declining — particularly for those who were vaccinated very early on — the FDA felt that there was not enough data supporting the effectiveness and benefit of extending boosters to everyone. Much of the current booster data is based on the safety and benefit in an elderly population.

As with the initial EUA approval of the Pfizer vaccine, the FDA committee wants to have enough data to be sure a third vaccine dose will benefit the broader American population. As the committee chair commented, “The beauty of an EUA is that it can be modified as more data comes in.” 

In the meantime, we know that Delta cannot spread effectively in a fully vaccinated population that’s taking the appropriate safety measures.

Which is why the most important things we can do as a community to fight the Delta variant are to:

  • Get everyone vaccinated
  • Wear masks while indoors
  • Avoid crowds and indoor gatherings during times of high community transmission

In addition to being eligible for a third dose under the support of the extended EUAs, your physician may prescribe a booster for you, based on his or her best judgment for off-label prescribing.

Talk to your doctor if you’re unsure whether you are eligible for a third dose.

Q: What’s a doctor-prescribed third dose?

Dr. Sostman: Because Pfizer’s COVID-19 vaccine is fully approved by the FDA, your physician can choose to prescribe the Pfizer vaccine as a booster, based on his or her best judgment for off-label prescribing.

Off-label means the vaccine is being administered outside of the FDA’s fully approved guidelines. It is very common for many different medications to be prescribed in this manner.

Q: Who is eligible for a third dose without a prescription right now?

Dr. Sostman: Third doses are available without a prescription as a:

  • Third shot in the primary vaccination series for immunocompromised people after 28 days have passed since his or her second shot
  • Booster shot at least six months after the primary vaccination series for certain people who are more vulnerable

Third Shot Eligibility

People who are immunocompromised are eligible for a third shot in the primary vaccination series after 28 days have passed since his or her second shot.

A person is considered immunocompromised if he or she:

  • Is actively being treated for solid tumor or blood-related malignancies
  • Has received a solid-organ transplant and is taking immunosuppressive medications
  • Has received a CAR T-cell or hematopoietic stem cell transplant in the last two years
  • Is actively being treated with certain immunosuppressive medications, including high-dose corticosteroids, alkylating agents, antimetabolites, transplant-related immunosuppressive drugs, cancer chemotherapeutic agents classified as severely immunosuppressive, TNF blockers and other biologic agents that immunosuppressive or immunomodulatory
  • Has a moderate or severe primary immunodeficiency, such as DiGeorge and Wiskott-Aldrich syndromes
  • Has advanced or untreated HIV infection

Booster Shot Eligibility

The following people should or may get a Pfizer booster shot:

  • Anyone who is 65+ and any long-term care resident should receive a Pfizer booster shot at least 6 months after his or her primary series
  • Anyone 50+ with an underlying medical condition should receive a Pfizer booster shot at least 6 months after his or her primary series
  • Anyone 18–49 with an underlying medical condition may receive a Pfizer booster shot at least 6 months after the primary series, based on his or her individual benefits and risks
  • Anyone 18-64 who is at increased risk of COVID-19 exposure and transmission because of job or institutional settings may receive a Pfizer booster shot at least 6 months after the primary series, based on his or her individual benefits and risks

Underlying medical conditions that may increase a person's risk of developing severe COVID-19 include:

  • Cancer
  • Stroke
  • Chronic kidney disease
  • Chronic lung diseases
  • Dementia
  • Diabetes
  • Heart conditions
  • Obesity

People who are pregnant and those who smoke are also considered high risk.

Visit the CDC's website for a detailed list of high risk conditions.

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Q: Is it important for your third shot or booster to match the original vaccine you received? Or can you mix and match?

Dr. Sostman: For third doses, the FDA recommends that attempts be made to match your third dose to the original mRNA vaccine you received.

This may not always be feasible, however. If this is the case, an additional dose of a different mRNA vaccine is permitted.

Finally, the FDA does not currently have sufficient data to support the use of an additional mRNA COVID-19 vaccine dose after a primary Johnson & Johnson COVID-19 vaccine in immunocompromised people. The FDA and CDC are actively working to provide guidance on this issue.

Q: Could getting a third shot or booster cause any harm?

Dr. Sostman: The CDC studied what happened when certain people were given a third dose of an mRNA vaccine. The symptoms reported were consistent with previous doses and the intensity of the symptoms was mostly mild or moderate. If anything, the reactions to the third dose were a little bit milder.


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