Vaccines for children: Vaccine safety, concerns and side effects

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With so much information available about vaccines it can be difficult to know which information is correct and can be trusted. Get the facts on vaccine safety and vaccine side effects to help address your concerns.

Vaccines do not cause autism

Contrary to what some people believe, there is no link between getting vaccinated and developing autism.

In 1998, The Lancet (a British medical journal) published a study conducted by former British doctor Andrew Wakefield. This study wrongly claimed that the measles, mumps, rubella (MMR) vaccine caused autism in young children.

Unfortunately, the study has been widely quoted since then. It was later found to be completely false and was retracted by The Lancet.

Since then, many larger studies have explored the same topic and tried to replicate the results. They have proven that there is no difference in the rate of autism between children with the vaccine and children without it.

Researchers and scientists around the world have since rejected any link between the MMR vaccine and autism.

Vaccine side effects

Like any medication or supplement (including vitamins), vaccines can cause side effects and reactions.

After being vaccinated, it’s common for your child to have mild (but harmless) side effects. These can last a few hours or days after vaccination.

This is the body’s natural response, as it’s working hard to build immunity against the disease. This is known as the inflammatory response or reaction. These reactions should not disrupt daily activities and can be treated if needed.

Common vaccine side effects may include:

  • a low fever
  • being fussy
  • being sleepier than usual
  • a stiff, slightly swollen or sore arm (or leg) where the needle went in

You can give your child medicine to help with the pain or lower the fever. Ask your healthcare provider what is recommended.

Vaccine reactions

There is a small chance of an allergic reaction to a vaccine (less than 1 in 1 million). In very rare cases this can be a serious reaction known as anaphylaxis. This type of reaction usually happens shortly after the vaccine is given.

This is why your healthcare provider will ask you to stay in the clinic for 15 to 20 minutes after your child’s vaccination. They will know how to treat your child if a reaction happens.

Your healthcare provider will then report the information to their public health department. This is to make sure abnormal or unexpected reactions are monitored and dealt with quickly.

Signs of a serious allergic reaction to a vaccine include:

  • swelling of the face
  • breathing problems (wheezing)
  • blotchy skin on the body (hives)

Once again, other serious reactions to vaccines are very rare. Call your healthcare provider or public health office (CLSC in Quebec) if your child has unusual symptoms after vaccination.

These symptoms may include:

  • unusual sleepiness
  • a fever above 40°C (104°F)
  • crying or fussing for more than 24 hours
  • worsening swelling where the needle went in

As a parent, you know your child best. If you see any of these symptoms, talk to your health care provider right away.

How vaccines are tested for safety

Before approving the use of a vaccine in Canada, it must go through a series of tests to make sure it’s safe. On average, it takes 10 years of research and development before a vaccine is considered for approval by Health Canada.

Even after vaccines are in use, their safety and effectiveness are regularly checked by people who give them, such as:

  • nurses
  • doctors
  • pharmacists

Their safety and effectiveness are also checked by:

  • pharmaceutical companies who make them
  • Health Canada scientists who review them
  • experts who analyze them
  • public health teams who monitor them

To learn more about the safety of vaccines, watch our vaccine safety video and read the vaccine safety poster.

Risk of getting the disease from a vaccine

Experts have developed different types of vaccines, including some that contain killed germs (inactive) and others that contain live but weakened germs (attenuated).

Inactive vaccines

Inactive vaccines, such as the polio vaccine, use inactive or killed germs in the vaccine development process. It is not possible to develop disease from vaccines that are made with inactive or killed germs (viruses and bacteria).

This is because the killed infectious agent cannot reproduce or multiply. However, the body’s defence system still recognizes these dead germs as a foreign body, stimulating an immune reaction.

Live vaccines

Live vaccines, like the MMR vaccine, use live germs that are weakened (attenuated) during the vaccine development process. Live vaccines are very effective because the weakened germs act like a natural infection, building up the body’s immune system without causing any serious symptoms.

Your child could experience very mild symptoms of the disease but this is rare. For example, unlike the severe symptoms of a natural measles infection, the weakened germs in the vaccine could result in a few spots on the skin or a mild to moderate fever. These mild symptoms are not dangerous and can actually demonstrate that the vaccine is working effectively to build immunity.

Natural immunity and disease prevention

Natural immunity occurs when a child becomes immune to a disease after being naturally infected through exposure to disease spreading in the community. However, if a child isn’t vaccinated and catches the disease, there is a real risk of severe complications or even death.

This is because germs multiply quickly. Your child’s immune system may not be prepared to fully defend itself.

Vaccines help get your child’s immune system ready to protect against a disease without making them sick.

The risk of complications from getting the real disease is far greater than the risk of side effects from vaccines. For example, if you catch measles naturally, you have a 1 in 1,000 chance of developing encephalitis (a severe, life threatening swelling of the brain), whereas your chance of developing encephalitis from the vaccine is less than 1 in 1 million.

Breastfeeding and disease prevention

Breastfeeding provides temporary protection by passing some of the mother’s immunity to the baby. Babies who are breastfed often have lower rates of:

  • diarrhea
  • ear infections
  • viral lung infections

However, breast milk is not a substitute for vaccination. It does not protect against all diseases that can be prevented by vaccines. Its protection is only partial and only for a short time. This is why it’s still very important to vaccinate your child even if they are being breastfed.

Vaccine ingredients

The main components in all vaccines are called antigens. Antigens are the weakened or killed germs used to teach the body’s immune system to recognize and attack the disease.

The antigens in vaccines cannot cause serious illness; rather they serve to protect your child if they come in contact with the real germ.

Vaccines contain minute amounts of other ingredients. Each ingredient serves a specific purpose. For example:


Formaldehyde is used in some vaccines such as hepatitis A vaccine. It is used during the vaccine development process to kill or disable the viruses or bacteria.

The human body naturally produces formaldehyde. An infant’s body contains about 10 times the amount of formaldehyde found in 1 dose of a vaccine. The tiny traces that may be found in the vaccine are safe.


Sometimes many doses of vaccine can be taken from the same vial. Thimerosal stops harmful bacteria and fungi from growing inside these multi-dose vaccine vials. Even though a new needle and syringe are used for each new dose, thimerosal adds another layer of protection. It prevents the vaccine from becoming accidentally contaminated which could cause serious infections in the people getting the vaccine.

Thimerosal is not used in single dose vaccine vials. Routine vaccines used for children in Canada come in single dose vials and are therefore thimerosal free.

Thimerosal breaks down into ethylmercury in the body and quickly leaves the body in the stool. It does not build up in the body and does not cause health concerns. Another type of mercury, methylmercury, is the type found in certain kinds of fish, like tuna. Unlike ethylmercury, it does not leave the body and can build up, causing harm.

Thimerosal has been explored in many well conducted studies. During its long history of use in preventing contamination of vaccines, thimerosal has never been found to cause any harm.


Aluminum salts are added to some vaccines to strengthen the body’s immune response to the antigens. This is known as an adjuvant.

Aluminum is one of the most common metals found in nature and is present in:

  • air
  • food
  • water

There is less aluminum in vaccines than the amount found in breast milk or infant formula. Hundreds of millions of people have been safely vaccinated with aluminum-containing vaccines.

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