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Last updated February 5, 2021. 5 minute read

What are the side effects of the COVID-19 vaccine?

The most common side effects of the COVID-19 vaccines include pain at the injection site, fatigue, and headaches. Read on for more on how the vaccine works and its potential side effects.

Dr Chimene Richa Md Written by Chimene Richa, MD
Reviewed by Mike Bohl, MD, MPH

Following the start of the coronavirus disease 2019 (COVID-19) pandemic, scientists worldwide have been working on vaccines to stem the spread of this contagious disease. 

In December of 2020, two companies, Moderna and Pfizer-BioNTech, received Emergency Use Authorization (EUA) from the U.S. Food and Drug Administration (FDA) to start vaccinating people in the United States. Initial clinical trials show a 94.5% effectiveness for the Moderna vaccine, and a 95% effectiveness for the Pfizer-BioNTech version (FDA, 2020a; FDA, 2020b). Since then, millions of Americans have safely gotten the COVID-19 vaccine (CDC, 2021a). 


  • The most common side effects of the COVID-19 vaccines include pain at the injection site, fatigue, and headaches. 
  • Severe allergic reactions, or anaphylaxis, are uncommon and most occur within 30 minutes of getting the vaccine.
  • Around 80% of people who have a severe allergic reaction to the vaccine have a history of allergies to foods or drugs.
  • The vaccine does not cause the COVID-19 illness, infertility, or changes in your DNA.

How do the COVID-19 vaccines work?

The vaccines work by training your immune system to recognize and attack the coronavirus before you get sick with COVID-19. It accomplishes this by delivering blueprints, or genetic code, for the spike proteins that live on the outer shell of the COVID-19 virus. 

Once your cells receive these blueprints, they get to work making those spike proteins. Since the vaccine only contains the information for these specific proteins, your cells can’t do anything else with the data. The immune system sees the spike proteins and responds to them, making antibodies to store for future use. When exposed to the virus, your immune system already has the ammunition it needs to fight off infection before it has a chance to make you ill. 

Common side effects of the COVID-19 vaccine

No medical therapy is perfect—all carry some risks of side effects. The important thing is to recognize this and weigh the risks and benefits before choosing a therapy. A healthcare provider can help if you have any questions. The Centers for Disease Control and Prevention (CDC) has ongoing monitoring of side effects of the vaccine via their v-safe program. As more and more people get vaccinated, scientists will continue to obtain data regarding the after-effects.

When it comes to the COVID-19 vaccine, most of the side effects are not serious and usually improve within a few days. Some people experience more side effects after the second dose, but everyone is different. Common side effects include (CDC, 2021b):

  • Pain at the injection site
  • Fatigue 
  • Headaches
  • Muscle aches
  • Chills 
  • Fever 
  • Swelling 
  • Joint pain 

While some of these side effects may make you feel sick” they are not the same as having COVID-19. These symptoms usually improve within a few days. The side effects are a sign that your immune system is responding to the spike proteins. The CDC has some tips to help you feel more comfortable after your vaccine (CDC, 2021c):

  • Put a clean, cool, wet washcloth over the part of your arm where you got your shot.
  • Move your arm around every once in a while.
  • Drink plenty of fluids and dress lightly if you’re feeling feverish.
  • Talk to a primary care doctor about taking over-the-counter medicine, such as ibuprofen or acetaminophen.

Severe allergic reactions

A potential serious side effect of the COVID-19 vaccine is the development of a severe allergic reaction. Also called anaphylaxis, a severe allergic reaction can include a variety of symptoms like trouble breathing, trouble swallowing, dizziness, low blood pressure, hives, and others. Anaphylaxis can occur with any vaccine or medication, although it is fortunately uncommon (AAAA1, 2020). 

Both of the currently available vaccines have caused severe allergic reactions. According to the CDC, approximately five cases of anaphylaxis occur per one million doses of the Pfizer-BioNTech vaccine and less than 3 cases per one million doses of the Moderna vaccine.  Within this very small group of people with severe allergic reactions, 80% had a history of allergic reactions to foods or drugs. Since 90% of anaphylaxis reactions occurred within 30 minutes of getting the vaccine, you will likely be monitored for 30 minutes after getting the vaccine to make sure that no serious reactions develop if you have a history of anaphylaxis (CDC, 2021d). 

If you think you might be having a severe allergic reaction to the vaccine, you should call 911.

Myths about side effects

Myth: Getting COVID-19 is a side effect of the COVID-19 vaccine

Since the COVID-19 vaccine does not contain live COVID-19 virus particles, you cannot get sick with COVID-19 from the vaccine. It just doesn’t have the tools to give you the disease. The vaccines only carry viral genetic code for the spike proteins and not the rest of the viral particle. Some of the side effects of the vaccine may make you feel ill, but they do not mean that you have COVID-19. It’s just your body’s way of telling you that your immune system sees the spike proteins, and is busy making antibodies. 

However, it does take time for your immune system to make antibodies to protect you against the virus. Therefore, you could get sick with COVID-19 shortly after getting the vaccine if you are exposed to the virus before your body has had time to build up immunity. But this is not because the vaccine gave you COVID-19. 

Myth: Changes in your DNA are potential side effects of the COVID-19 vaccine

The current COVID-19 vaccines do not change your DNA. The vaccines developed by Moderna and Pfzer-BioNTech carry viral mRNA. Even though it is genetic material, mRNA is different from DNA and the two cannot be combined. Your DNA lives in the nucleus of your cells, while mRNA stays outside of the nucleus in the cytoplasm, where your protein-making factories also live. The whole purpose of the mRNA is to deliver genetic information to these factories so they can manufacture the viral spike proteins. Once this purpose is fulfilled, the mRNA is broken down by your cell’s natural processes. The viral mRNA does not affect your DNA (CDC, 2021e).

Myth: Infertility is a side effect of the COVID-19 vaccine

Infertility (the inability to have children) is not a side effect of the vaccine. There is false information circulating on the internet that reports that the viral spike protein is similar to a protein (syncytin-1) needed for fertility—so alike that people think your body will form antibodies against syncytin-1, leading to infertility in women. 

However, this is not true because the two proteins are not alike enough for this to occur. Also, consider this—if antibodies against the COVID-19 virus caused infertility, then many women who were exposed to COVID-19 would be infertile. There is no evidence to suggest that this has occurred and during the vaccine clinical trials, several women even became pregnant (Pfizer, 2021).