Vaccines are safe and effective interventions that can help prevent infectious diseases. They are biological products that typically contain part of a pathogen that the immune system can use to provide immunity. However, as vaccinations activate the immune system and typically require an injection, some people may experience mild side effects.

Vaccinations are critically important for public health, as they help prevent disease and save lives. All vaccines can cause side effects, but these are generally minor, such as a low grade fever or pain at the injection site. Some people may also experience other side effects, depending on the type of vaccine and the disease against which it is protecting.

However, it is worth noting that researchers continually monitor vaccines for safety. Anyone uncertain about a vaccine’s side effects or other possible interactions in their body should talk with their doctor to discuss all possibilities.

In this article, we explain what vaccines are and discuss the potential side effects of common vaccinations.

An infant with an adhesive bandage over the injection site of a vaccine.Share on Pinterest
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A vaccine is a biological product that prompts the body to develop immunity to a pathogen without getting an infection. Vaccinations typically achieve this by using either an inactive form of the pathogen, part of the pathogen, or just the genetic material of the pathogen. Each of these approaches allows the body to recognize the pathogen and stimulate the immune system to prepare an immune response to fight off the infection at a later date.

The World Health Organization (WHO) notes that vaccines are available to protect against more than 20 diseases and that they save an estimated 2–3 million lives each year. Researchers are also continually developing vaccines to protect against infections for which there are currently no vaccines.

Scientists have developed several types of vaccines to fight off pathogens. Each type uses a slightly different approach to provide the human body with immunity. The types of vaccines include:

  • Live-attenuated vaccines: These vaccines use a weakened, or attenuated, form of the pathogen that causes a disease. Overcoming the weakened version of the pathogen can help provide a strong and long lasting immune response. Typically, 1–2 doses of these vaccines may create lifetime protection. However, they may not be suitable for people with a weakened immune system, and doctors must keep the vaccine cool.
  • Inactivated vaccines: Inactivated vaccines use a killed version of an infectious pathogen. Inactivated vaccines usually do not provide such strong protection as other vaccines, so a person may require booster shots.
  • Messenger RNA (mRNA) vaccines: Using the pathogen’s genetic material, known as mRNA, this type of vaccine instructs the cells to make a harmless protein that is present on the surface of the pathogen. The immune system can then recognize the protein and develop a response that protects against future infections. These vaccines are typically quick to manufacture and safe for people with a compromised immune system.
  • Toxoid vaccines: Toxoid vaccines use a toxin that a pathogen makes to provide the body with immunity to the parts of the pathogen that cause disease. Toxoid vaccines may only remain effective if a person gets booster shots.
  • Subunit, recombinant, polysaccharide, and conjugate vaccines: This group of vaccines uses specific parts of a pathogen, such as a sugar or protein, which gives a strong immune response to these particular components. Booster shots may be necessary, but these vaccines are suitable for most people, including those with immune system complications.
  • Viral vector vaccines: This type of vaccine uses an altered version of a safe virus — which is unlikely to cause any illness — as a vector to deliver the vaccine to the body. Exposure to the vector prompts the immune system to develop a successful way to combat the infection.

The possible side effects of a COVID-19 vaccine will depend on the specific vaccine, but they may include:

  • pain, discoloration, or swelling at the injection site
  • tiredness
  • headaches
  • muscle pain
  • chills
  • fever
  • nausea

Learn more about specific COVID-19 vaccines and their possible side effects by clicking on the links below:

Learn more about remedies for COVID-19 vaccine side effects.

There are many vaccines available in the United States that doctors recommend for both children and adults to help prevent disease and save lives. Any side effects that a person may experience are typically mild — such as a fever, sore arm, body aches, and feeling tired — and they usually go away in a few days.

Although they rarely occur, adverse reactions to some vaccines are possible. A person may wish to discuss all possibilities with their doctor before having a vaccine.

After receiving a vaccine, a person should tell their doctor if they feel faint, have vision changes, or hear ringing in the ears. Additionally, anyone who experiences difficulty breathing, a rash covering the body, or swelling in the throat after a vaccination should seek medical assistance.

Of the vaccines that doctors regularly recommend, possible side effects may include:

Haemophilus influenza type b (Hib) vaccine

After a haemophilus influenza type b vaccination, a person may experience discoloration, warmth, and swelling at the injection site. Fever is another possible side effect.

Hepatitis B vaccine

If a person experience side effects from a hepatitis B vaccine, these may include soreness at the site of injection and fever.

Human papillomavirus (HPV) vaccine (Gardasil 9)

Possible side effects from the Gardasil 9 vaccine for HPV may include headaches and fever. People may also experience soreness, discoloration, and swelling at the injection site.

Influenza vaccination

Influenza vaccination typically involves live or inactivated vaccines, which can cause slightly different side effects.

For the live vaccine, side effects may include:

  • runny nose
  • nasal congestion
  • wheezing
  • headaches
  • vomiting
  • muscle aches
  • fever
  • cough
  • sore throat

For the inactivated influenza vaccine, side effects may include:

  • soreness, discoloration, or swelling at the injection site
  • fever
  • muscle aches
  • headaches
  • a very small increased risk of Guillain-Barré syndrome

Young children who receive an inactivated flu shot along with the PCV13 pneumococcal vaccine and/or the DTaP vaccine for diphtheria, tetanus, and pertussis may be slightly more likely to have a seizure due to fever.

Measles, mumps, and rubella (MMR) vaccine

The side effects of an MMR vaccine can include:

  • soreness or discoloration at the injection site
  • rash at the injection site or spread over the body
  • fever
  • swelling of the glands in the cheeks or neck

Rare reactions may include:

  • seizure, often associated with fever
  • temporary pain and stiffness in the joints
  • pneumonia
  • swelling of the brain or spinal cord covering
  • temporary unusual bleeding or bruising due to a low platelet count

As with other live-attenuated vaccines, this vaccine may not be suitable for people with an impaired immune system, who may be at risk of an infection that could be life threatening.

Pneumococcal vaccines

There are two common pneumococcal vaccines: the conjugate vaccine (PCV13) and the polysaccharide vaccine (PPSV23).

Potential side effects of the PCV13 vaccine include:

  • discoloration, swelling, pain, and tenderness at the injection site
  • fever
  • loss of appetite
  • irritability
  • fatigue
  • headaches
  • chills

Young children may have an increased risk of seizures resulting from fever if they receive PCV13 at the same time as an inactivated influenza vaccine.

Possible side effects of a PPSV23 vaccine include:

  • discoloration and pain at the injection site
  • fever
  • fatigue
  • muscle aches

Polio vaccine

After a polio vaccine, a person may experience a sore spot with discoloration, swelling, and pain at the injection site.

Rotavirus vaccine

Possible side effects of the rotavirus vaccine include:

  • irritability
  • mild diarrhea
  • vomiting
  • a small risk of intussusception in infants

Tetanus, diphtheria, and pertussis booster (Tdap) vaccine

Possible side effects of the Tdap vaccine include:

  • pain, discoloration, or swelling at the injection site
  • mild fever
  • headaches
  • tiredness
  • nausea
  • vomiting
  • diarrhea
  • stomachache

Vaccinations are a safe and effective medical intervention that can prevent disease and save lives. As with any form of medical intervention, vaccines may cause side effects. However, these side effects are usually minor and only last for a few days.

Anyone who would like to report side effects from a vaccine can do so using the Vaccine Adverse Event Reporting System from the Department of Health and Human Services. This system relies on individuals to report their experiences and adverse events from any given vaccination.