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What is a COVID booster shot, and do I need one?

yael coopermangina-allegretti

Reviewed by Yael Cooperman, MD, written by Gina Allegretti, MD

Last updated: Jan 13, 2022
6 min read


Information about the novel coronavirus (the virus that causes COVID-19) is constantly evolving. We will refresh our novel coronavirus content periodically based on newly published peer-reviewed findings to which we have access. For the most reliable and up-to-date information, please visit the CDC website or the WHO’s advice for the public.

COVID-19 vaccines are a crucial part of our effort to curb the spread of the coronavirus and reduce serious infections. But the antibodies you build up after getting vaccinated don’t last forever. When your protection starts to go down, it might be time for a booster shot. 


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What are booster shots? 

Like tetanus and flu shots, COVID vaccines need booster doses to support the immunity you develop after your initial vaccine series. This doesn’t mean your vaccine didn’t work. 

On the contrary, your initial vaccines enable your immune system to develop protective antibodies and train cells in your immune system to fight a specific infection. When you are exposed to a virus, your immune system has to spend time building up protection to allow you to fight off the virus and recover. With vaccines, you can train your system without ever getting sick. 

Vaccines work like little mugshots for your immune system. They wave a picture of the virus in front of your immune cells so they know what to look for, without the need to cause infection in the first place. That allows your body to build up an army of immune cells and antibodies to protect you from that infection. But our systems are built in a way that conserves energy. If you aren’t exposed to the mugshot (the vaccine) or the virus itself, your body spends less time maintaining that army of protective cells and molecules. 

Boosters remind your immune system to maintain that protection. They give your immune system a “boost” so it can keep protecting you as efficiently as possible. As long as coronavirus is prevalent, we may need to continue getting boosters. Also, some scientists believe that we’ll need a once-a-year booster shot (like the annual flu vaccine) to stay protected in the future. 

What’s in a COVID booster shot? 

If you’ve been vaccinated, you’ll know what to expect with a booster. All doses of the COVID booster are made of the same ingredients as your initial COVID vaccine. The only difference is that the Moderna booster uses half of a dose since it’s as effective as a full one (FDA-a, 2021; CDC-a, 2022). 

The Pfizer and Moderna vaccines contain a small amount of a molecule called mRNA. This molecule is like a postcard with instructions on it that your cells can use to make the virus’s spike proteins, allowing your immune system to learn to recognize the virus without being infected. 

Why get a booster? 

Many of us get flu shots once a year and tetanus shots every 5–10 years. So why is the COVID booster being given so soon? 

Each disease and the vaccine used to protect us from that disease has a unique impact on our immune systems. That means we can stave off some diseases longer than others. Before the spring of 2021, research showed that the available COVID vaccines were more than 90% effective in preventing severe disease. But once the Delta variant emerged and more people got breakthrough infections, it became clear that immunity can decrease over time (CDC-a, 2021). 

Researchers performed studies to understand that protection better. One study of over 4,800 people found that six months after getting vaccinated, levels of protective antibodies dropped significantly. This was especially true in high-risk groups, including people over the age of 65, people with underlying medical conditions, or people who were taking medications that suppressed their immune systems (Levin, 2021). A booster dose is used to improve protection when your initial antibodies drop. 

Further research from Pfizer-BioNTech and Moderna also found that booster shots increased antibody levels and improved protection against severe illness, hospitalization, and death from COVID (Moderna-a, 2021; Pfizer, 2021). As a result, booster doses are now a standard recommendation. 

Who is eligible for a COVID booster? 

The recommendations for booster doses differ slightly depending on the vaccine you already received. The Centers for Disease Control and Prevention (CDC) recommends the following: Pfizer-BioNTech and Moderna vaccines 

All people over age 12 who got Pfizer-BioNTech or Moderna vaccines should get a booster (CDC-a, 2022). The recommendations are:

  • People over age 18 can get either a Pfizer-BioNTech or Moderna booster.
  • People ages 12-17 can only receive the Pfizer-BioNTech booster. 
  • People ages 5-11 do not currently need a booster.
  • People with weakened immune systems should get an additional dose of the vaccine to complete their primary series (CDC-b, 2022). This means that they will receive three full vaccine doses rather than two. They are eligible for a booster dose (4th dose) 28 days after completing the primary series.

People with weakened immune systems include people who have had cancer, bone marrow transplants, or organ transplants; people with HIV; and people on medications that affect the immune system. 

If you’re not sure which booster you should get, consult with a healthcare provider or pharmacist. 

If you are not in the high-risk group, after getting your second dose of the Moderna or Pfizer vaccine, wait at least five months before getting a booster shot.Johnson & Johnson’s Janssen vaccine 

All people over age 18 who got J&J’s Janssen vaccine should get a booster. After getting your initial vaccine, wait at least two months before getting a booster (CDC-a, 2022). 

Which booster should you get? 

According to the CDC, it’s ok to mix and match boosters. This means you don’t have to get the same type of booster as your initial vaccine––as long as you’ve completed your initial vaccine series (two doses of Pfizer-BioNTech or Moderna, or one dose of the Janssen vaccine) (CDC-a, 2022). 

One study even found that mixing and matching may increase participants’ protective antibodies as much as giving the same type (Atmar, 2021). While this study isn’t published yet, the results are promising.  

Side effects of the COVID booster 

So far, side effects from the COVID booster shot are the same as side effects from the COVID vaccine

The most common reaction is pain and swelling at the injection site. Some people experience fever and chills, mild to moderate aches and pains, headaches, fatigue, or swollen lymph nodes under the arm. These symptoms usually resolve on their own within a few days (CDC-c, 2022; FDA-a, 2021). 

Serious side effects from the vaccine or a booster shot are rare. Anaphylaxis, a severe allergic reaction, can happen after any of the vaccines, usually within a half-hour of receiving one. Blood clots are a rare but serious side effect seen in people who received J&J’s vaccine, particularly in women under age 50. Heart inflammation is another rare reaction following the mRNA vaccines. All of these are uncommon, and the risks of severe COVID are considered much more dangerous than any adverse effects experienced from a vaccine or booster (CDC-b, 2021). 

Where to get a COVID booster 

Some healthcare providers can give you a booster in the office, but it’s always a good idea to call and check with them first. If not, many large chain pharmacies like Walgreens and CVS offer appointments for booster doses. 

Vaccines have been one of the most important tools we have of slowing the spread of COVID and inching closer to the end of the pandemic. If you are over age 12, you’re eligible for a booster shot to enhance protection against the virus. If you aren’t certain when to get a booster, ask a healthcare professional for advice. 


  1. Atmar, R. L., Lyke, K. E., Deming, M. E., Jackson, L. A., Branche, A. R., El Sahly, H. M., et al. (2021). Heterologous SARS-CoV-2 Booster Vaccinations – Preliminary Report. MedRxiv. doi:10.1101/2021.10.10.21264827. Retrieved from
  2. Centers for Disease Control and Prevention (CDC-a). (2022). COVID-19 Vaccine Booster Shots. Retrieved on Jan. 12, 2022 from
  3. Centers for Disease Control and Prevention (CDC-b). (2022). COVID-19 Vaccines for Moderately or Severely Immunocompromised People. Retrieved on Jan. 12, 2022 from
  4. Centers for Disease Control and Prevention (CDC-c). (2022). Possible Side Effects After Getting a COVID-19 Vaccine. Retrieved on Jan. 12, 2022 from
  5. Centers for Disease Control and Prevention (CDC-a). (2021). Effectiveness of COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Frontline Workers Before and During B.1.617.2 (Delta) Variant Predominance — Eight U.S. Locations, December 2020–August 2021. Retrieved on Jan. 12, 2022 from
  6. Centers for Disease Control and Prevention (CDC-b). (2021). Safety of COVID-19 Vaccines. Retrieved on Jan. 12, 2022 from 
  7. Levin, E. G., Lustig, Y., Cohen, C., Fluss, R., Indenbaum, V., Amit, S., et al. (2021). Waning Immune Humoral Response to BNT162b2 Covid-19 Vaccine over 6 Months. The New England Journal of Medicine, 385, e84. doi:10.1056/NEJMoa2114583. Retrieved from
  8. Moderna-a. (2021). Moderna Announces FDA Authorization of a Booster Dose of Moderna’s COVID-19 Vaccine in the U.S. Retrieved on Oct. 28, 2021 from
  9. Moderna-b. (2021). Moderna Highlights New Clinical Data on its COVID-19 Vaccine. Retrieved on Oct. 28, 2021 from
  10. Pfizer. (2021). Pfizer and BioNTech Announce Phase 3 Trial Data Showing High Efficacy of a Booster Dose of their COVID-19 Vaccine. Retrieved on Oct. 28, 2021 from
  11. U.S. Food and Drug Administration (FDA-a). (2021). Coronavirus (COVID-19) Update: FDA Takes Additional Actions on the Use of a Booster Dose for COVID-19 Vaccines. Retrieved on Jan. 12, 2022 from
  12. U.S. Food and Drug Administration (FDA-b). (2021). FDA Authorizes Booster Dose of Pfizer-BioNTech COVID-19 Vaccine for Certain Populations. Retrieved on Jan. 12, 2022, from

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.