What are the vaccines’ side effects?

By Mayo Clinic Staff

May 11, 2022

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Why do the COVID-19 vaccines cause side effects?
Find out about the side effects of the COVID-19 vaccine and why you should get it.

Melanie Swift, M.D., COVID-19 Vaccine Allocation and Distribution, Mayo Clinic: When we get vaccinated, we often experience some side effects and the reason that we get side effects is that our immune system is revving up and reacting. Now when you get sick, the same thing happens and actually a lot of the symptoms from illnesses that we get like influenza and COVID, are actually caused not by the direct action of the virus, by our immune system, so our bodies react and that gives us these general symptoms like fever, achiness, headache.

When you take two doses of vaccine, the first vaccine is the first time for your body to see this particular protein, the spike protein that the vaccines produce and your body begins to develop an immune response. The second vaccine dose goes into your body, start to make that spike protein and your antibodies jump on it and rev up and your immune system responds.

The vaccine side effects that we've seen resolve within about 72 hours of taking the vaccine. At the most, those side effects can last up to a week. And we really have not seen long-term side effects from the vaccine beyond that. It is so important to get this vaccine when it's offered to you, even if you're healthy, even if you might not be at risk for complications from COVID yourself. You could catch it and transmit it to other people who are and our society really needs everyone to be vaccinated so that we can stop transmission.

Key takeaways

COVID-19 vaccines can cause mild side effects after the first or second dose, including:

  • Pain, redness or swelling where the shot was given
  • Fever
  • Fatigue
  • Headache
  • Muscle pain
  • Chills
  • Joint pain
  • Nausea and vomiting
  • Swollen lymph nodes
  • Feeling unwell

Most side effects go away in a few days.

In the U.S., there has been an increase in reported cases of myocarditis and pericarditis after mRNA COVID-19 vaccination, particularly in males ages 12 through 29. Myocarditis is the inflammation of the heart muscle, while pericarditis is the inflammation of the lining outside the heart. These reports are rare.

Of the cases reported, the problem happened more often after the second dose of the COVID-19 vaccine and typically within one week after COVID-19 vaccination. Most of the people who received care felt better after receiving medicine and resting. Symptoms to watch for include:

  • Chest pain
  • Shortness of breath
  • Feelings of having a fast-beating, fluttering or pounding heart

If you or your child develops myocarditis or pericarditis after a dose of an mRNA COVID-19 vaccine, the CDC recommends avoiding getting another dose of any COVID-19 vaccine.

The Janssen/Johnson & Johnson COVID-19 vaccine can cause thrombosis with thrombocytopenia syndrome. This is a blood-clotting problem that can be life-threatening.

As a result, the FDA is restricting use of the Janssen/Johnson & Johnson vaccine to certain people age 18 and older. Examples include people who had a severe allergic reaction after getting an mRNA COVID-19 vaccine and people who can’t get an mRNA COVID-19 vaccine due to limited access or personal or religious concerns. Research suggests there isn’t an increased risk of this blood-clotting problem after mRNA COVID-19 vaccination.

Serious side effects of the Janssen/Johnson & Johnson COVID-19 vaccine can occur within three weeks of vaccination and require emergency care. Possible symptoms include:

  • Shortness of breath
  • Persistent stomach pain
  • Severe or persistent headaches or blurred vision
  • Chest pain
  • Leg swelling
  • Easy bruising or tiny red spots on the skin beyond the injection site

This vaccine shouldn’t be given to anyone who develops thrombosis with thrombocytopenia after getting the Janssen/Johnson & Johnson COVID-19 vaccine or any other adenovirus vector COVID-19 vaccine.

Mild to moderate headaches and muscle aches are common in the first three days after vaccination and don’t require emergency care.

Some people who received the Janssen/Johnson & Johnson COVID-19 vaccine have developed Guillain-Barre syndrome. The chances of this happening are very low. This disorder is most often reported within 42 days after vaccination, mostly in men. Many of the men are ages 50 to 64. Seek immediate medical care if you have weakness or tingling sensations, difficulty walking, difficulty with facial movements, double vision, and difficulty with bladder control.

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