5 Questions About Getting Your Third Dose of COVID-19 Vaccine, Answered



Katie McCallum

Given that 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 many whom are seriously immunocompromised.

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

A booster, on the other hand, is the term used for a third dose that’s needed to help prolong protective immunity in someone who responded fully after the first two doses but there’s evidence that protection is waning after some time.

Because the FDA recently granted full approval to Pfizer’s COVID-19 vaccine, a licensed health care provider can now 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.

Pfizer is planning to ask for full approval of a third dose as a booster shot in the coming weeks. Currently, a third shot of Pfizer or Moderna is only authorized under EUA for immunocompromised individuals. A third dose of Moderna as a booster is not yet available to non-immunocompromised people.

Q: Who is considered immunocompromised and should receive a third shot?

Dr. Sostman: Third shots are available immediately without a prescription — but at least 28 days after the second vaccine shot — for immunocompromised individuals who meet the criteria below:

  • Are actively being treated for solid tumor or blood-related malignancies
  • Have received a solid-organ transplant and are taking immunosuppressive medications
  • Have received a CAR T-cell or hematopoietic stem cell transplant in the last two years
  • Are 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 and TNF blockers
  • Have a moderate or severe primary immunodeficiency, such as DiGeorge and Wiskott-Aldrich syndromes
  • Have advanced or untreated HIV infection

Talk to your doctor if you’re unsure whether you are considered immunocompromised and might benefit from a third shot.

Q: Has the Delta variant affected the need for third shots and boosters?

Dr. Sostman: As the highly infectious Delta variant continues to spread, experts believe a third shot will increase antibody protection in immunocompromised people. This allows physicians to boost immunity in certain immunocompromised individuals who need extra protection from COVID-19 — and does not apply to those who are not immunocompromised.

For most people, the vaccines still provide excellent protection against severe illness and death.

What new data is showing, however, is that protection against mild and moderate disease is reduced, and that protection against severe disease and hospitalization is likely to diminish in the coming months — especially among those who are high-risk.

Given this, as well as how well Delta spreads, boosters are being talked about more and more as a way of staying ahead of the virus. This is probably due to a combination of waning antibody levels and the very high viral load produced by the Delta variant.

Pending regulatory approval, third doses will be made available to everyone beginning the week of September 20 and starting 8 months after a person’s second dose — even if you’re not immunocompromised. However, your physician may choose to prescribe the Pfizer vaccine as a booster sooner than that, based on his or her best judgment for off-label prescribing.

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

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 shots, the FDA recommends that attempts be made to match your third dose to the original mRNA vaccine you received. However, this may not always be feasible. If this is the case, an additional dose of a different mRNA vaccine is permitted. This additional dose should be administered at least 28 days after completion of the primary mRNA COVID-19 vaccine series, however.

As for boosters prescribed by a physician, the Houston Methodist Vaccine Scientific Committee recommends they match the original vaccine. However, if your primary vaccine series was Moderna, a third dose of Pfizer is acceptable.

At the present time, the FDA does not 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 immunocompromised patients were given a third shot of an mRNA vaccine. The symptoms reported were consistent with previous doses and the intensity of the symptoms was mostly mild or moderate.

As for booster shots, both Pfizer and Moderna submitted data to the FDA indicating that the safety and the post-inoculation side effects are similar after a third dose. Further follow up is ongoing.