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Understanding vaccines


Vaccination is the best way to protect yourself and others against certain diseases.

Vaccines protect us against diseases with serious consequences and that can even cause death. Some of these illnesses have no medical treatment. By getting vaccinated, you also reduce the transmission of contagious diseases.

In Québec, getting vaccinated is not mandatory but is highly recommended. The vast majority of people do get vaccinated. The Québec Immunization Program [Programme québécois d’immunisation] provides free vaccines to the entire population of Québec according to an immunization schedule.

Make sure your vaccines are always up to date. See the Québec Immunization Program to find out how to get vaccinated

To protect yourself and avoid the risks and complications of illnesses

Vaccine-preventable diseases cause suffering, complications and after-effects. They may also cause death. Catching certain diseases naturally offers subsequent protection against these diseases, in the same way that vaccines do. However, letting nature take its course presents very great risks to your health. Here are a few examples:

  • Meningococcal infection, which causes meningitis, is a serious disease. It can cause permanent brain damage and be fatal in 10-15% of cases
  • Chickenpox is a common disease that is not dangerous most of the time. However, it can lead to complications such as skin and ear infections and pneumonia. In 2006, the chickenpox immunization program was established in Québec. In the six years that followed, hospitalisation due to chickenpox complications dropped by 85%
  • A communicable disease such as measles spreads very easily from one person to another. This disease can be serious or even fatal in people who are at higher risk of developing complications from measles. For examples, very young children, people with a weak immune system and pregnant women are more vulnerable to complications of measles. To prevent this disease from spreading in the population, at least 95% of the public must be vaccinated

Vaccination also carries some risks, but the serious risks associated with vaccines are much rarer than those related to diseases. For example, after receiving the measles vaccine, there is a risk of encephalitis, which is an inflammation of the brain. However, this risk does not even represent 1 case in 1 million. This risk increases to 1 in 1,000 cases when a person catches measles.

To protect the people around you

People who are vaccinated against a disease are less likely to catch it. As a result, they are less likely to transmit it to others. Hence, they indirectly protect those who have not received the vaccine.

It is therefore important to get vaccinated in order to protect others. Indeed, some people cannot get vaccinated for one or more of the following reasons:

  • Allergy to the content of certain vaccines
  • Their age
  • Their health (a weak immune system, for example)

Also, some people remain inadequately protected even if they are vaccinated.

Therefore, they’ll always be people not protected against a particular disease. That’s why as many people as possible need to be vaccinated in order to prevent transmission of diseases and to protect the entire population.

See the Objectives of vaccination page for further information on the objectives of COVID-19 vaccination.

To prevent the resurgence of vaccine-preventable infectious diseases

With the advent of vaccines, some infectious diseases have become rare and others have disappeared. If there were no more vaccines, infectious diseases would reappear quickly and spread through the population. This has happened in some countries. For example, in 2015, there was a measles outbreak in the United States and Canada because a significant number of people were not vaccinated.

Despite some infectious diseases having become rare in Québec, vaccine-preventable diseases are still present. For example, tetanus will continue to exist because it is caused by a bacterium that lives in the soil.

In addition, certain vaccine-preventable diseases are very common in several countries. People travelling can catch and spread them upon their return. Therefore, it is important to continue to be protected against vaccine-preventable diseases.

From birth, the human body defends itself every day against thousands of microbes in water, air, food and on objects. Through evolution, the body has developed a defence mechanism known as the ‘immune system' to protect itself against microbes. The immune system’s function is to detect intruders entering the body, such as microbes, and to eliminate them.

Vaccine ingredients

Vaccines are made from small amounts of microbes responsible for certain diseases. These microbes are processed in order to deactivate their ability to transmit diseases. However, they are still able to stimulate the immune system into learning to defend itself against those particular diseases. The processed microbes that contain the vaccines may be:

  • Entire dead or weakened bacteria or viruses
  • Portions of dead or weakened bacteria or viruses

Vaccines currently distributed in Canada also contain various other ingredients, including:

  • Culture media, which serve to multiply the viruses and bacteria used in the production of vaccines, chicken eggs, for example
  • Suspending fluids that hold all ingredients of the vaccine in liquid form, for example, a saline liquid or sterile water
  • Inactive substances that facilitate the preparation and administration of vaccines. Examples of inactive substances include:
    • Preservatives or antibiotics, which prevent the rapid multiplication of bacteria in the vaccine
    • Stabilisers, which prevent viruses or bacteria from being destroyed during production of the vaccine. They also prevent them from sticking to the walls of vaccine vials, which would make them less effective
    • Adjuvants, which are used to strengthen vaccines and ensure longer protection against diseases

Ways to administer vaccines

Vaccines can be administered through various methods. Methods vary depending on the vaccine.

  • Injectable vaccine is introduced into the body through injection with a syringe
  • Intranasal vaccine is introduced into the body through a spray in the nose, a squirt in each nostril
  • Oral vaccine is drunk or swallowed

How the immune system responds to vaccines

Once in the body, the weakened microbes or parts of the microbes contained in the vaccine cause the immune system to react.  

As it is the first time that the immune system encounters these microbes, it produces antibodies, or a type of protein, specifically to fight them.

Once produced, these antibodies remain in the system for a period that varies depending on the vaccine. Some vaccines offer protection for a lifetime or very close to that, the hepatitis B vaccine is one such example. Other vaccines offer protection for a much shorter period, the flu vaccine, for instance.

When the system encounters these microbes again, the antibodies are able to quickly recognise them given their ability to ‘remember’ the tactics employed to fight them. In other words, it's a bit as if the body made a replica image of the virus or bacterium in order to recognise it and respond quickly next time. If the virus or bacterium that actually causes the disease enters the body after vaccination, the immune system is able to defend itself rapidly and can destroy the threat before it becomes dangerous.

Inflammatory reaction

Vaccination causes an inflammatory reaction. Heat, redness and swelling at the injection site are common reactions to vaccines.

Such reaction is neither abnormal nor a sign of illness. On the contrary, it is a defense mechanism, a bit like a gathering of the troops around the action. Thus, the inflammatory reaction is to isolate and protect the area of the body at risk from viruses or bacteria contained in the vaccine. It triggers a mechanism that directs as much blood as possible to this area, which allows the different blood cells to fight together to avoid infection.

In addition, the inflammatory reaction forces neighbouring cells in the threatened area to swell with water in order to create a barrier of protection. Hence the reason why there is often swelling at the site of the injection. 

Effectiveness of vaccination

Like all other medicines, no vaccine is 100% effective. The effectiveness of a vaccine depends on the following:

  • The age of the person vaccinated
  • The health condition of the person vaccinated (pregnancy, state of immune system)
  • Relatedness of strains of virus circulating and those contained in the vaccine

Despite this, immunization remains one of the greatest medical successes. It is one of the most effective medical interventions. The World Health Organization estimates that immunization saves over 2 million lives each year. In Canada, the number of sick people has significantly declined since the introduction of immunization programs in 1920.

Through vaccination, smallpox was eradicated from the planet. Polio has disappeared from Canada and several diseases, including diphtheria, tetanus and rubella, are now very rare. The main cause of bacterial meningitis among children, the bacterium Haemophilus influenzae type b, is much rarer. In addition, hepatitis B has virtually disappeared among younger children because they are vaccinated as infants.

Safety of vaccination

Vaccines are very safe. They are produced according to very strict safety standards. However, like many medicines, vaccines can cause side effects.

Most often, the side effects of vaccines are minor and temporary. It could be a light fever or sensitivity at the site of the injection. These effects are normal reactions of the body to the vaccine.

In very rare cases, vaccination can cause serious allergic reactions, in the same way that an allergy can occur when you eat a new food. People administering vaccines are able to treat these allergic reactions. That’s why it is recommended you stay onsite at least 15 minutes after receiving a vaccine.

Research and surveillance programs

In Québec and elsewhere worldwide, side effects of vaccines are constantly monitored. Surveillance helps detect unexpected, serious and rare side effects, and to intervene as needed. Thus, when vaccinators discover unusual reactions to a vaccine, they must report it to public health authorities.

One such surveillance scheme in Québec is the ESPRI program or ‘side effects possibly caused by immunization program’ (Effets secondaires possiblement reliés à l’immunisation). This Québec program is in line with the Public Health Agency of Canada’s program, as well as the World Health Organization’s international program.

Through research, vaccines continue to improve. For example, the vaccine used in the 1950s against whooping cough has been replaced with one that causes far fewer side effects.

There are many beliefs about the supposed risks of immunization. To help you better understand, read Demystifying beliefs regarding the risks of vaccination.

Vaccine injury compensation program

In Québec, anyone who believes they have been injured by a vaccine can file a claim for compensation with the Ministère de la Santé et des Services sociaux. To find out more, consult the Vaccine injury compensation program page.

Last update: June 13, 2023


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