COVID-19 Vaccine Side Effects: Why They Happen and How to Treat Them

MSK Chief Medical Epidemiologist Mini Kamboj

MSK Chief Medical Epidemiologist Mini Kamboj gets vaccinated.

Like with any vaccination, there is potential for side effects after you get your shot. The good news is that the COVID-19 vaccines have similar side effects that are typically mild and don’t last long — about one to three days.

The most common side effects include a sore arm, fatigue (feeling tired), headache, aches, and fever. Severe side effects are very rare and treatable.

Will I experience side effects?

It is difficult to predict. Side effects are more common, and may be more pronounced, after the second dose of Pfizer-BioNTech COVID-19 vaccine (Comirnaty®) or Moderna COVID-19 vaccine (Spikevax™).

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What do the side effects mean?

If you get side effects, they are a good sign — they indicate that the vaccine is working by triggering the immune system.

When you get vaccinated, your immune system recognizes something as being foreign. The immune system automatically launches a small-scale attack against it. This process teaches your immune cells to recognize and respond to an “invader.” That’s why you might experience some side effects. Think of it this way: The body’s response to the vaccine is like a training mission for the real fight.

Once you’re fully vaccinated, if you were infected by the virus causing COVID-19, your immune system would be ready to launch an even larger and more powerful attack to protect you.

If you don’t experience any side effects, that doesn’t mean that the vaccine didn’t work. In the vaccine clinical trials, more than half of people didn’t experience any side effects but we still know that the vaccine was effective in those people.

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Can I treat the side effects?

If you have pain or discomfort after receiving your vaccination, talk to your doctor about taking an over-the-counter medicine, such as ibuprofen (Advil) or acetaminophen (Tylenol).

To reduce pain and discomfort on your arm:

  • Apply a clean, cool, wet washcloth over the area.
  • Use or exercise your arm.

In most cases, discomfort from fever or pain is normal. Contact your doctor or healthcare provider:

  • If the redness or tenderness where you got the shot increases after 24 hours.
  • If your side effects are worrying you or do not seem to be going away after a few days.
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Can I do anything ahead of time to reduce any side effects from the vaccine?

You should wait until after being vaccinated to see how you feel. If you do experience side effects, it is OK to take an over-the-counter drug (like Advil or Tylenol) to lower a fever, reduce chills, or relieve a headache or body aches. It’s important that you do not take these drugs before getting your vaccine as there are theoretical concerns that some pain relieving drugs may interfere with the immune response to the vaccine. It is also unclear if taking medication ahead of time actually works to reduce post-vaccine symptoms.

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Are there any possible severe side effects I should know about?

A few extremely rare instances of severe side effects have emerged and been investigated, but scientists have concluded the risk of getting severely ill from COVID-19 is far greater than the risk of these side effects. Here is proof of this careful and ongoing scrutiny:

  • In mid-July 2021, the CDC said that about 100 out of 13 million Americans who received the J & J vaccination developed Guillain-Barré syndrome. Guillain-Barré syndrome is a rare neurological condition in which the body’s immune system attacks nerve cells. Read more »
  • In late June 2021, the CDC reported cases of heart inflammation potentially linked to the Pfizer-BioNTech and Moderna vaccines. The heart issues were very rare. In the clinical trial testing the Novavax COVID-19 vaccine, there were also some cases of heart inflammation. Read more »
  • As of late November 2021, the CDC and FDA had identified 54 people who were diagnosed with a blood clotting disorder called thrombocytopenia syndrome (TTS) after receiving the J & J vaccine (among 16.4 million doses). On December 16, the CDC recommended that Americans receive either the Pfizer-BioNTech or Moderna vaccine rather than the J & J vaccine. Read more »
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What if I experience swelling or tenderness away from where I was injected?

Some people may experience some swelling or tenderness in their lymph nodes. It is also possible that this swelling will show up on imaging tests and could be mistaken for progression of  certain cancers — primarily breast, head and neck, melanoma, and lymphoma.

This vaccine side effect is more common after the second dose, usually occurs within 2 to 4 days after the vaccination, and can last for an average of 10 days.

On imaging tests, the lymph node enlargement may be detected for a longer period. For these reasons, we recommend:

  • If you develop this symptom after you’re vaccinated, you should speak to your doctor. Most of the time, they will recommend that you wait at least four weeks before getting further tests, to give time for the swelling to disappear.
  • You should schedule your COVID-19 vaccination after any routine imaging. If you’ve already had the vaccine, then we recommend you wait six weeks for any routine breast screenings, including mammography and breast MRI.
  • If you’ve had cancer, you should ask for your COVID-19 vaccine to be administered on the opposite side of your cancer diagnosis if possible.
  • If you have any discomfort from the swelling, you can use a warm compress. Acetaminophen or non-steroidal anti-inflammatory drugs may be taken to ease the discomfort.

It’s important to know that all kinds of vaccines can cause temporary swelling of the lymph nodes, which may be a sign that the body is making antibodies in response as intended.

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Should I get an antibody test after the vaccine to make sure it’s working?

Antibody testing is not currently recommended check someone’s immunity to COVID-19 following vaccination. The COVID-19 antibody test used at MSK detects the immune response after being infected by COVID-19. It does not measure immunity that results from the vaccine. That’s why it should not be routinely ordered to assess vaccine response.

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Is it still possible to get COVID-19, even after being vaccinated? How does the effectiveness compare with vaccines for other diseases?

The COVID-19 vaccines are among the most effective vaccines in history. They are as effective — if not more — than vaccines for polio, chicken pox, measles and the flu.

The chances of getting sick after vaccination are minimal. Studies show even if you develop COVID-19 after being vaccinated, you are unlikely to get severely ill. Flu vaccines are less effective than the COVID vaccines, but they can protect you from more severe flu illness and hospitalization. The COVID-19 vaccines are even more powerful.

Learn more about the COVID-19 vaccines » 


August 8, 2022


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