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What You Need to Know About COVID Boosters

This information has been reviewed and approved by Carrie Horn, MD (October 2021).

Sometimes we all need a little reminder. The same could be said about our immune systems when it comes to fighting COVID-19 after initial vaccination.

If the booster shot is the exact same vaccine as the first two, why do you need a third? 

patient receiving COVID booster

Carrie Horn, MD, chief medical officer and chief of the Division of Hospital & Internal Medicine at National Jewish Health, likens getting the booster to reminding our immune systems that we still have protection.

“It’s keeping it in your short term memory, even though your long term memory still has it. You obviously remember what kind of car you have driven for the past year, but sometimes you forget where you parked this morning,” she said. “The booster gives the immune system’s short-term memory a boost.”

Or more literally, it will give your body a quicker, more robust response to the virus, rather than having to wait to wake up that long-term memory.

Below are answers to frequently asked questions about the booster.

"The booster gives the immune system’s short-term memory a boost."

- Carrie Horn, MD


Who Needs a Third Dose and When?

On August 12, 2021, the U.S. Food and Drug Administration (FDA) amended the emergency use authorizations for the Pfizer-BioNTech and Moderna COVID-19 vaccines to allow for an additional dose, following an initial two-dose, for certain individuals who are severely immunocompromised. On August 13, 2021, the Centers for Disease Control and Prevention (CDC) outlined recommendations for protecting those most vulnerable to COVID-19.

Currently the CDC recommends that people who are moderately to severely immunocompromised receive an additional dose. This includes people who:

  • ImmunocompromisedHave been receiving active cancer treatment for tumors or cancers of the blood
  • Have received an organ transplant and are taking medicine to suppress the immune system
  • Have received a stem cell transplant within the last two years or are taking medicine to suppress the immune system
  • Are moderate or severe primary immunodeficiency (such as DiGeorge syndrome, Wiskott-Aldrich syndrome,
  • Have advanced or untreated HIV infection
  • Are receiving active treatment with high-dose corticosteroids or other drugs that may suppress your immune response

“Right now, the focus is on people who may not have responded well to the first two doses. We want to give them the best possible chance of having a good immune response first,” said Horn. “Then we can roll it out to people who are getting into that eight months or more time window, where their immunity may be waning just because of how long it’s been since their body saw that second dose.”

If you are immunocompromised and received the Johnson & Johnson vaccine, there is not enough data to determine whether you need an additional dose. Watch the CDC’s website for an update or talk with your doctor.

Benefits of the Third Dose

Benefits of the Third DoseThe research trials around the vaccine suggest there may be decreased effectiveness over time. The third dose is designed to re-prime your immune system to provide better immunity. People who have compromised immune systems may benefit from an additional dose because they are more at risk of serious, prolonged illness and may not have developed the same level of immunity from the two-dose vaccine as people with healthy immune systems.

Horn adds that though the vaccine still matches the Delta variant, the variant is just so much more infectious. “Your body can likely still respond to the virus eventually, but this third dose will give it that boost earlier so that you’re less likely to get symptomatic disease,” she said.

People who are moderately to severely immunocompromised are especially vulnerable to COVID-19, but even with the third dose, it is still important for them to take preventive measures to help prevent catching COVID-19 – wash hands frequently, cover coughs and sneezes with a tissue or elbow, wear masks indoors and stay physically distant especially from those you appear to be sick. Make sure your close contacts are vaccinated as appropriate.

When to Get the Third Dose

If you qualify based on CDC guidelines, you may receive a third dose COVID-19 vaccine. If you are severely immunocompromised, you can get a third dose as early as one month after the second dose. Booster doses are recommended 6-8 months after antibody levels begins to wane for the other groups – frontline workers, individuals age 65 or older and those with chronic illness/immune suppression. These guidelines are the same for both the Pfizer and Moderna COVID-19 vaccines. There is currently no guidance on getting an additional dose of the Johnson & Johnson COVID-19 vaccine.


Mixing Vaccine Types Doesn’t Work

While there have been some studies suggesting this is acceptable, mixing vaccine types/doses (Moderna and Pfizer, etc.) is not recommended, except in extenuating circumstances.

“The problem is they weren’t big studies and we can’t endorse it without it being studied in more detail,” said Horn. “So to be safe, we want patients to stick with the same vaccine type they have already received.”

The CDC states that a person should not receive more than three mRNA COVID-19 doses of any type.

Third Dose Side Effects

Reported reactions so far are similar to that of the initial two doses – fatigue and pain at the site of injection were the most commonly reported, with most symptoms mild to moderate. The information on risks of the third dose is currently limited and continues to be evaluated.

Healthy Immune Systems and a Third Dose

People who have healthy immune systems and are not immunocompromised may need a third dose of vaccine, but there is no recommendation from public health officials yet. Watch the CDC and FDA for updates over the next several weeks.

“We would like you to get the booster as soon as it’s available, but if you are healthy, it’s OK to wait a little bit,” Horn said. “The original vaccinations still work. You still have immunity. The booster will just give you a little added support that will last longer.”
Talk to your health care provider about your medical condition, and whether getting an additional dose is appropriate for you.




The information on our website is medically reviewed and accurate at the time of publication. Due to the changing nature of the COVID-19 pandemic, information may have since changed. and your state’s health department may offer additional guidance.

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