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What are the side effects of vaccines?

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Common Questions-doctor and young boy high five

The most common side effects of vaccines are:source: 1

  • Pain, swelling, or redness where the shot was given
  • Mild fever
  • Chills
  • Feeling tired
  • Headache
  • Muscle or joint aches

These side effects can be a sign that your child’s immune system is working with the vaccine.source: 2 They are temporary and should go away within a few days.source: 3 You can put a cool cloth on the spot where the vaccine was given to help with soreness. You can also ask your child’s doctor about giving fever-reducing medicine if needed.

Most of the common side effects are mild and go away quickly on their own.

Severe allergic reactions are rare—about 1 or 2 cases for every 1 million vaccinations.source: 1,source: 4 Severe allergic reactions will typically happen very quickly after vaccination, often when a child is still in the doctor’s office. A few signs of a severe allergic reaction to a vaccine are:source: 1

  • Difficulty breathing
  • Face or throat swelling
  • Dizziness
  • Weakness
  • Fast heartbeat
  • Bad rash all over the body

Learn more about the side effects of each vaccine in their Vaccine Information Statement.

Key Evidence

  • Serious side effects are rare. Research shows that for most vaccines, if 1 million doses of a vaccine are given, between 1 to 2 people may have a severe allergic reaction.source: 1,source: 4,source: 5
  • Developmental disorders are not a side effect of vaccines. In 2019, a study looked at nearly 660,000 children over 11 years and found no connection between autism spectrum disorder (ASD) and measles, mumps, and rubella (MMR) vaccines.source: 6
  • Nearly all side effects show up within 45 days of getting a vaccine.source: 7 We know this based on more than 100 years of research across many different vaccines. For instance, a rare side effect of the MMR vaccine is a temporary condition with decreased platelets (blood cells that aid in clotting). This disease is called thrombocytopenia. Someone is more likely to get this condition if they have measles or mumps than if they get the vaccine. If this side effect does occur, it usually happens between 7 to 21 days after vaccination or, in rare cases, up to 6 weeks after vaccination.source: 8
  • Side effects can be the same for the disease and the vaccine—but with the vaccine, they are much less likely and don’t last as long. For example, common symptoms of the flu include fever, headache, and fatigue that can last up to 2 weeks. All of these are also common after a flu vaccine, but they typically only last 1–3 days if they even happen at all.source: 9
  • Side effects of vaccination are usually mild and will go away on their own. For instance, the most common side effect of the flu shot is some pain at the injection site. Occasionally people can get a fever. Side effects typically only last 1–3 days if they even happen at all.source: 3
  • A vaccine now can better protect your child in the long run, even if they have mild side effects from the vaccine. For example, most children still attend school even if they have a side effect after getting vaccinated against chickenpox, as most side effects are mild.source: 10 However, if your child gets chickenpox, then they will need to be kept home from school and away from other children for about 5 to 6 days.source: 11 Two doses of chickenpox vaccine prevents more than 90% of cases.source: 12

Your doctor or pharmacist would not give vaccines to your child if the side effects were worse than the diseases they prevent. Experts carefully study vaccines. Their goal is to do what is best for your child.

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Common Questions- doctor with baby

A Deeper Dive: Learning more about side effects

Fevers after a vaccination

Fevers are one of the most common side effects of a vaccination.source: 3,source: 13 It is understandable that you may be worried if your child gets a fever after a vaccination. But a fever after vaccination can be a sign the vaccine is working.source: 2,source: 14 A fever from a vaccination usually lasts a day or two at most.source: 3 And it is less severe than complications from the disease.

If your child is experiencing a fever, you can ask your child’s doctor about giving them a fever-reducing medicine.source: 15 Learn more about how to keep your child comfortable after a vaccine appointment.

What to do if your child has a serious side effect

Most serious side effects will take place within minutes of getting a vaccine.source: 16 In the rare instance your child has a severe allergic reaction hours later, call 9-1-1 or go to the nearest hospital. Talk with your child’s doctor if you are concerned about your child’s health after a vaccine. You or their doctor can choose to report the side effect to the Vaccine Adverse Event Reporting System.

Vaccines do not cause autism

Over the years, many studies have looked for a link between vaccines and ASD. Research continues to show that vaccines do not cause ASD.source: 17

  • Some people wonder about ASD and thimerosal, which is in some vaccines. Many studies have looked at thimerosal and have found no link between thimerosal in vaccines and ASD.source: 18,source: 19 Multidose vaccines (vaccines where more than one dose is in a vial) have thimerosal added to prevent germs from growing. Today, it is not included in childhood vaccines, with the exception of some flu vaccines that are in multi-dose vials. Flu vaccines are currently available in both thimerosal-containing and thimerosal-free versions. Parents can ask for a flu vaccine that does not contain this preservative.source: 20
  • In 2019, a study looked at nearly 660,000 children over a period of 11 years. Researchers did not find any connection between ASD and the MMR vaccine.source: 6
  • Studies also continue to find no link between ASD and children receiving multiple vaccines at once.source: 21

Signs of ASD

The signs of ASD often appear in children between 12 to 18 months of age. This falls within the first 2 years of a child’s life. This is also when most children receive several routine childhood vaccines. This might cause some parents to think that vaccines are linked to ASD. But people with ASD have unique brain activity, structures, and connections even at very young ages.source: 22 ASD symptoms can be seen as early as 12 months old.source: 23 Doctors, scientists, and researchers continue to work to understand all the causes of ASD, but vaccines are not one of them.source: 17

Find information about specific vaccines and the most common side effects.

Back to the common questions

More information

Still have questions? Talk to your child's doctor, nurse, or pharmacist.

Sources

  1. HHS: Vaccine Side Effects
  2. NHLBI: Vaccine Side Effects Signal their Effectiveness
  3. CDC: Possible Side Effects from Vaccines
  4. Journal of Allergy and Clinical Immunology: Risk of anaphylaxis after vaccination in children and adults
  5. CDC: Safety Information by Vaccine
  6. Annals of Internal Medicine: Measles, Mumps, Rubella Vaccination and AutismHviid A, Hansen JV, Frisch M, Melbye M. Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Ann Intern Med. 2019 Apr 16;170(8):513-520. doi: 10.7326/M18-2101.
  7. National Education Association: Dr. Anthony Fauci Answers Educator Questions About COVID-19
  8. CDC: Measles, Mumps, Rubella (MMR) Vaccine Safety
  9. CDC: Pink Book: Influenza
  10. CDC: Chickenpox (Varicella) Vaccine Safety
  11. CDC: Chickenpox Symptoms and Complications
  12. CDC: Pink Book: Varicella
  13. Children’s Hospital of Philadelphia: Fever and Vaccines
  14. WHO: Side Effects of COVID-19 Vaccines
  15. MedlinePlus: Fever
  16. CDC: Preventing and Managing Adverse Reactions
  17. CDC: Autism and Vaccines
  18. CDC: Science Summary: CDC Studies on Thimerosal in Vaccines
  19. FDA: Thimerosal and Vaccines
  20. CDC: Thimerosal and Vaccines
  21. CDC: Multiple Vaccines at Once
  22. NIH: Autism Spectrum Disorder (ASD)
  23. CDC: About Autism Spectrum Disorder
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