Here's the Difference Between COVID-19 Booster Shots and Third Doses

Someone receiving a COVID vaccine.

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Key Takeaways

  • At the moment, only certain people who got the Pfizer-BioNTech vaccine are eligible to get a booster shot.
  • A booster vaccine dose is for individuals who already built COVID-19 immunity from vaccination but whose protection may have waned over time.
  • On the other hand, an additional vaccine dose is intended to help immunocompromised individuals build adequate protection from COVID-19.

As Pfizer booster shots roll out nationwide, vaccinated folks who received Moderna and Johnson & Johnson are still awaiting their turn. But you may have heard of people getting Moderna "boosters" although the shots are not yet authorized.

While some people are finding loopholes in the vaccination system, most aren't skirting rules and receiving boosters at all. In fact, people who are getting a third dose of the Moderna shot are getting what is considered an "additional dose" of the vaccine. Turns out, there's a difference between the two.

The distinction between boosters and additional COVID-19 doses lies in the purpose of the shot for the person receiving it. Here’s how you can differentiate them.

What’s the Difference Between a Booster and Additional Doses?

People often use the terms “booster” and “additional dose" interchangeably. However, they mean two different things.

“Vaccination doses can be described as being part of the ‘prime’ series or a ‘booster’ dose,” Richard Martinello, MD, Yale Medicine infectious diseases specialist and associate professor at the Yale School of Medicine, tells Verywell. “The purpose of the prime series is to prime the immune system to develop immunity against the target pathogen, while the purpose of the booster dose is to boost the already existing immunity to ensure continued protection against the pathogen.”

For individuals who aren't immunocompromised, two doses of the Pfizer-BioNTech and Moderna vaccines and one from the Johnson & Johnson vaccine are enough to give them immunity from COVID-19. Since their protection against infection wanes over time, “booster” doses may then be recommended.

However, moderately to severely immunocompromised individuals may not build the same level of immunity with one or two COVID-19 vaccine doses. This means that an “additional” dose is necessary to help them build adequate protection.

“For those severely immunocompromised who were recommended to receive an additional dose of mRNA vaccine a few months ago, ‘additional dose’ is the correct phrase as this was recommended to ensure they developed immunity, [which is] part of the prime series,” Martinello says. “It would be incorrect to call this third dose for those severely immunocompromised a ‘booster dose’ as research showed they required at least a three-dose prime series to develop this initial immunity.”

Recipients of the mRNA vaccines who are not immunocompromised have already built COVID-19 immunity from their two-dose vaccine series. So, folks who are getting an additional dose of Moderna are likely immunocompromised and need that extra shot to mount immunity against the virus.

“For people with compromised immune systems, a third dose [of the mRNA vaccines] has been recommended for a number of months, and this dose can be given as early as one month after the second dose,” David Dowdy, MD, epidemiologist at the Johns Hopkins Bloomberg School of Public Health, tells Verywell.

The Centers for Disease Control and Prevention (CDC) started recommending booster doses of the Pfizer-BioNTech COVID-19 vaccine for certain populations in late September. Recipients of the Moderna and Johnson & Johnson vaccines are not yet eligible for a booster dose.

What This Means For You

If you are a recipient of the Moderna COVID-19 vaccine, you are not eligible to get a booster shot just yet. However, if you are immunocompromised, you would need to get your third dose at least 28 days after your second dose.

Who Is Eligible For Boosters and Additional Doses?

Booster doses of the Pfizer-BioNTech vaccine are available for the following individuals at least six months after their second dose:

  • Older adults aged 65 years and older
  • Adults with underlying medical conditions
  • Adults that are long-term care setting residents
  • Adults with a high risk for COVID-19 exposure due to their nature of work

Additional doses of the Pfizer-BioNTech and Moderna vaccine are available at least 28 days after the second dose for people who are moderately or severely immunocompromised due to the following:

  • Cancer treatments
  • Intake of immunosuppressive medications following an organ or stem cell transplant
  • Active treatment with other immunosuppressive medications such as high-dose corticosteroids
  • Moderate or severe primary immunodeficiency such as DiGeorge syndrome or Wiskott-Aldrich syndrome
  • Advanced or untreated HIV infection

The CDC does not recommend immunocompromised people receive both a booster and an additional dose of a COVID-19 vaccine at this time. There is no guidance yet on whether immunocompromised recipients of the Johnson & Johnson vaccine should get an additional dose of any COVID-19 vaccine.

“If you do not have any immune-compromising conditions and have not gotten a booster shot, you are still fully vaccinated,” Dowdy says. “If you are immunocompromised, however, you should not consider yourself fully vaccinated unless you get a third dose.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. COVID-19 Vaccines for Moderately to Severely Immunocompromised People.

  2. Centers for Disease Control and Prevention. CDC Statement on ACIP Booster Recommendations.

  3. Centers for Disease Control and Prevention. Who Is Eligible for a COVID-19 Vaccine Booster Shot?.

By Carla Delgado
Carla M. Delgado is a health and culture writer based in the Philippines.