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COVID-19 Booster Shots | Ask An Expert

COVID-19 Booster Shots | Ask An Expert

With SMH Infectious Disease Specialist Manuel Gordillo, MD

** NOTE: This content was originally posted Aug. 27, 2021. Information related to COVID-19 and COVID-19 vaccines is continually evolving. For the most up to date info, we recommend visiting

The U.S. Food and Drug Administration (FDA) recently authorized third shots of the Pfizer-Biotech and Moderna COVID-19 vaccines for people who are moderately to severely immunocompromised, and federal public health officials have announced that COVID-19 booster shots for all fully vaccinated people likely will be authorized in September. 

If vaccines are effective, why do we need another dose? What’s the difference between a COVID-19 “booster” and a “third shot” of the mRNA vaccines? Who’s eligible now for an additional shot? 

For answers to these and other common questions about vaccine boosters, we turned to Sarasota Memorial Infectious Disease Specialist Manual Gordillo, MD, who’s been on the frontlines of Sarasota’s COVID-19 fight since Day 1. Here’s what he had to say.

Third dose vs. booster: What’s the difference?

The Centers for Disease Prevention and Control (CDC) and the FDA are particular about the terminology with these. 

The “third shot” is for people who are moderately to severely immunocompromised. Because of weakened immune systems, this group is likely to have a weaker immune response to vaccination, which may prohibit them from getting the vaccine’s full protection in the first 2 doses.

COVID-19 “booster shots,” however, are for everyone else fully vaccinated (ages 12 and older). A booster shot is intended to strengthen the protection provided by the initial 2 shots as those antibodies begin to wane over time.

Recommendations for third shots and boosters come amid an unprecedented and deadly surge of COVID-19 cases in Florida, largely dominated by the contagious Delta variant, and reports of breakthrough cases among people who are fully vaccinated.

Who can get a third shot of the COVID-19 vaccine now?

Currently, only people who are moderately to severely immunocompromised are eligible to get a third dose of the Moderna or Pfizer vaccines. Making up about 3% of the U.S. population, this group includes people who: 

  • are in active treatment for cancer.
  • have received an organ or stem-cell transplant.
  • have advanced HIV infection.
  • are on hemodialysis for end-stage renal disease.
  • are being treated with high-dose corticosteroids or other drugs that suppress immune response.
  • take prednisone in moderate to high doses (more than 20 milligrams).

Anyone in this group should get a third vaccine dose now. They are the most at risk of a breakthrough infection.

The virus naturally tries to find the people it can easily infect. It’s smart and will find the easiest targets, which tend to be elderly people and those who are immunocompromised because they were unable to build adequate protection with the first 2 shots.

How long after my first COVID-19 vaccine series should I wait to get a third shot?

For people with weakened immune systems, the recommendation is to wait at least 28 days after your second injection of the Moderna or Pfizer vaccine to get an additional dose — unless you have received the monoclonal antibody treatment. 

Monoclonal antibody treatments can neutralize the spike protein in COVID-19 vaccines, rendering them ineffective. Anyone who has received monoclonal antibody treatment should wait 90 days after the treatment to get a COVID-19 vaccine injection.

Because the regulation process has not yet been completed, booster-shot dosing schedules have not been confirmed for people who are not immunocompromised.

Where can I get a third dose?

Once you’re eligible, you can get third shots wherever the Moderna (age 18 and older) or Pfizer (age 12 and up) vaccine is available. This includes many retail pharmacies like Publix, CVS and Walgreens, as well as Department of Health vaccination sites and some clinics. Click here for to find a vaccination site near you.

Is it OK to “mix and match” vaccines from 1 dose to the next? Or should I stick with the same vaccine brand for the third shot/booster?

If possible, get a third dose or booster shot of the same vaccine you received previously. For example, if your first 2 injections were of the Moderna vaccine, plan to get the Moderna vaccine for your third/booster shot.

But if you cannot get the same brand COVID-19 vaccine, it is safe to get an alternate.

People who received the single-injection Johnson & Johnson (Janssen) vaccine should note that the FDA has not yet authorized that vaccine for booster shots, and the CDC does not recommend getting Moderna/Pfizer boosters for people who had the J&J vaccine.

Also, now that the Pfizer vaccine has received full FDA approval, you may see it referred to by its commercial name: Comirnaty. As is standard when medicines or vaccines earn full FDA approval, Pfizer has since branded its COVID-19 vaccine as "Comirnaty." There have been no changes to the mRNA vaccine’s formula, ingredients or effectiveness; it simply has a new, branded name.

If vaccines are working, why do I need another shot?

The vaccines are doing what they're supposed to do, which is preventing recipients from severe infections, preventing them getting from sick enough that they end up in the hospital and preventing them from dying of COVID-19.

Breakthrough infections that lead to hospitalization or death are still very rare in the U.S. We are seeing breakthrough cases among those who are fully vaccinated, but they are mostly mild to moderate cases.

We do know from some studies that as time goes by, we're going to need booster shots. Over time, the vaccine’s protection is reduced because the antibodies your body made to fight this virus start decreasing.

When they were first rolled out, the vaccines offered about 94% protection. But, now with the Delta variant and with the decrease in antibodies from vaccines given 8 or 9 months ago, we're down to about 80% protection. So, boosters may be necessary for adequate protection now. 

Pfizer/Moderna booster shots are expected to be authorized in mid-September, once the regulatory process is complete. And by about Sept. 20, some of us will be needing boosters: the elderly, healthcare workers and people who live in nursing homes or other congregate living facilities — those who were first to get COVID-19 vaccines in December 2020 and January 2021.

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Posted: Aug 27, 2021,
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