COVID-19 Vaccine: What You Need to Know About Third Doses and Booster Shots

Photo illustration of COVID-19 vaccine vials

The first COVID-19 vaccine became available in late 2020, and since then more 50% of the total U.S. population has been fully vaccinated.

Now, the Centers for Disease Control and Prevention recommends that moderately to severely immunocompromised people who have completed a two-dose series of an mRNA vaccine should consider getting a third dose of the same vaccine.

Additionally, the CDC recommends Pfizer "booster" shots for frontline workers, as well as for people older than 65 and many people with underlying health conditions. This doesn't apply to people who had the Moderna series or the single-dose Johnson & Johnson shot.

What is the difference between a third dose and a booster shot?

A third dose of an mRNA vaccine is recommended for people who are immunocompromised, and it should be given at least 28 days after the second dose. The third dose is recommended because there is evidence that this group doesn’t build up the same level of immunity after two doses.

A booster shot, on the other hand, is intended to offer a boost of immunity a certain period of time after original vaccination. There is evidence that the effectiveness of the vaccine wanes over time. As for the timing of this shot, federal officials have discussed recommending it eight months after the second dose.

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Why should immunocompromised people consider a third shot?

People who are considered to be moderately to severely immunocompromised are especially susceptible to COVID-19 because they're at greater risk of serious illness, according to the CDC.

Some research indicates that immunocompromised people aren't protected by the vaccine to the same degree as people with strong immune systems, so they might benefit from an additional dose to maximum protection against COVID-19.

Although immunocompromised people make up less than 3% of the U.S. population, they have accounted for 44% of hospitalized breakthrough cases. They also might be more likely to transmit the virus to close contacts such as family members.

Who is considered moderately to severely immunocompromised?

Moderately to severely immunocompromised people meet any of the following criteria:

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

If you fit any of the above categories, you should talk to your doctor about your medical condition and whether you should consider a third shot.

What are the criteria for a third shot?

The CDC recommends a third shot only for those who completed a vaccine series of the mRNA vaccines — either Pfizer or Moderna. An additional shot is not yet recommended for those who received the single-dose Johnson & Johnson vaccine.

The third shot should be administered at least 28 days after you received your second dose of Pfizer or Moderna, and you should get the same brand, if available. If the same brand vaccine is not available to you, the CDC says you can get the other mRNA vaccine.

The CDC says a person should not receive more than three mRNA vaccine doses.

Why are booster shots recommended?

The protection of some vaccines lessens over time as the body’s antibody levels drop. In those cases, an extra dose of vaccine can refill the antibody tank and boost the immune response. That’s why booster shots are recommended for tetanus, for example.

The COVID-19 vaccines remain effective in reducing risk of severe disease, hospitalization and death, even against the Delta variant. However, the CDC has found that the COVID-19 vaccines’ protection against mild to moderate disease decreases over time. Based on that information, the vaccines’ protection against severe disease, hospitalization and death also could diminish in the months ahead.

What are the criteria for a booster shot?

The CDC recommendation applies to people who have had the Pfizer vaccine series, and the CDC says the booster shot should be administered at least six months after the second shot of the initial series.

The recommendation covers:

  • people 65 years and older and residents in long-term care settings should receive a booster shot.
  • people aged 50–64 years with underlying medical conditions should receive a booster shot.
  • people aged 18–49 years with underlying medical conditions may receive a booster shot, based on their individual benefits and risks.
  • people aged 18-64 years who are at increased risk for COVID-19 exposure and transmission because of occupational or institutional setting may receive a booster shot, based on their individual benefits and risks.
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