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COVID-19 Vaccine Safety

Learn about the safety of COVID-19 vaccines, including the approved Pfizer-BioNTech and Moderna vaccines, how they work and possible side effects.


The coronavirus (COVID-19) vaccine does not cause a coronavirus infection. It helps to build up your immunity to the virus, so your body will fight it off more easily if it affects you.

This can reduce your risk of developing coronavirus and make your symptoms milder if you do get it. The effectiveness and immune response of the vaccine is being monitored as the vaccine is rolled out. Vaccination is expected to be an effective way to prevent the spread and reduce the impact of COVID-19.

Only vaccines that Health Canada determines to be safe and effective will be approved for use in Canada and available in Ontario.

After independent and thorough scientific reviews for safety, efficacy and quality, Health Canada has approved two vaccines for use in Canada:

  • Pfizer-BioNTech – approved on December 9, 2020

  • Moderna – approved on December 23, 2020

This means the vaccines:

  • were tested on a large number of people through extensive clinical trials

  • have met all the requirements for approval, including safety

  • will be monitored for any adverse reactions that may occur after vaccination and appropriate measures will be taken

Both vaccines require two doses for your body to develop adequate immunity. After two doses, they are expected to be 94-95% effective, which will lower how much of the virus can spread in the population, help build herd immunity and stop the pandemic.

They were both manufactured in Belgium. Learn more about the vaccines from Health Canada’s website.

Vaccine development

Creating a new vaccine typically takes years. However, the progress on COVID-19covid 19 vaccines is happening quickly for many reasons, including:

  • being informed by decades of research on other strains of coronavirus prior to COVID-19covid 19, such as Middle East Respiratory Syndrome (MERS) and Sars-CoV (SARS)

  • advances in science and technology

  • international collaboration among scientists, health professionals, researchers, industry and governments

  • increased dedicated funding

Before any vaccines are available in Ontario, they:

  • undergo rigorous clinical trials to ensure they are safe and effective

  • are evaluated and authorized for use by Health Canada, using rigorous standards

Ontario’s plan to make sure vaccines remain safe for Ontarians includes:

  • securely and safely transporting and storing vaccines at required conditions and temperatures

  • establishing safe clinic spaces to give people immunizations, including providing the required training to those administering vaccines

  • monitoring for any adverse reactions or side effects that may occur after vaccination and taking appropriate measures, including working with the federal government and other provinces and territories

Health Canada will continue to monitor all authorized vaccines to ensure they continue to be safe and effective. Read more information on vaccines and vaccine authorization updates from the Government of Canada.

Vaccine ingredients and how they work

COVID-19 vaccines work by training your body’s immune system to recognize and fight the virus that causes the disease (SARS-CoV-2). To do this, the vaccine uses certain molecules or parts of the virus — called antigens — which trigger an immune response when they are introduced into the body through vaccination.

By injecting these antigens into the body, your immune system safely learns to:

  • recognize the antigens

  • produce antibodies to fight the antigens

  • remember the antigens for the future

If the virus reappears, your immune system will recognize the antigens and attack them before the disease can develop and cause sickness.

In addition to the antigens, vaccines can also include:

  • adjuvants (for example, aluminum salts) – help boost the body’s response to the vaccine

  • antibiotics – prevent contamination during the manufacturing process

  • preservatives and stabilizers – keep the vaccine stable, effective and safe when it’s being made, shipped and stored

Vaccine types

There are several different types of vaccines being studied to fight COVID-19covid 19. Three key types include:

mRNA vaccines

mRNA vaccines are a new type of vaccine. mRNA vaccines teach our cells how to make a protein from the virus that will trigger an immune response and create antibodies. These antibodies help us fight the infection if the real virus does enter our body in the future as they recognize this protein.

The Pfizer-BioNTech and Moderna vaccines that Health Canada approved for use are both mRNA type vaccines.

Viral vector-based vaccines

These vaccines use genetically modified viruses (vectors) that are harmless to humans.

Once injected into the body, the viral vector contained in the vaccine produces a part of the SARS-CoV-2 virus, such as its spike protein (the part of the virus that binds to our cells and starts the COVID-19 infection). The vector virus is not the virus that causes COVID-19 and doesn't make you sick. It does its job and then goes away.

This process triggers an immune response against the spike protein without exposing you to the virus that causes COVID-19.

Viral vector-based vaccines have been used to develop:

  • many vaccines for animals

  • an Ebola vaccine approved by a number of international regulators

Virus-like particle vaccines

Virus-like particles are molecules that mimic viruses but are not infectious.

They are very similar to real viral molecules, so introducing them into the body through vaccination triggers an immune response, without any symptoms of the virus they are being vaccinated against.

Once the body responds to the virus-like particles, it recognizes the virus and prevents infection in the future, giving people immunity to that particular virus.

Virus-like particles have been an effective way of vaccinating against diseases such as:

  • human papillomavirus (HPV)

  • hepatitis B

  • malaria

Possible side effects

Serious adverse reactions to vaccines are extremely rare. They happen less than one time in a million.

Once a vaccine is in use, Canada has a strong vaccine safety monitoring system to alert public health authorities of changing trends or unusual adverse events that were not previously reported.

The side effects observed during the clinical trials for the approved vaccines are very similar to other vaccines. They were all mild or moderate and included things like pain at the site of injection, body chills, feeling tired and feeling feverish.

The vaccines cannot cause COVID-19. This is because they do not contain the SARS-CoV-2 virus responsible for the disease. However, if you come in contact with the virus just before or after you complete the vaccine series of two doses, you could still develop COVID-19.

To date, no serious adverse effects have been identified with the vaccines approved for use.

If you do have a severe adverse reaction to a COVID-19 vaccine, you may be eligible for compensation from the federal government.

Read Health Canada’s recommendations for people with serious allergies.

Approving vaccines in Canada

Drugs, including vaccines, are regulated under the Food and Drugs Act and regulations. They must meet the regulatory requirements for safety, efficacy and quality before they can be approved for use and distribution in Canada. The federal government (Health Canada) is responsible for approving vaccines.

Before authorizing a vaccine, Health Canada assesses the:

  • scientific and clinical evidence — including results of clinical trials — to determine if a vaccine product is safe, effective and manufactured to the highest quality

  • safety and efficacy of the vaccine to determine that there are no concerns, the vaccine can trigger an adequate immune response to protect against disease and the benefits outweigh the risks

  • the manufacturing process to make sure the manufacturer can carry out the necessary quality controls for the vaccine

If there is not enough evidence to support the manufacturer’s safety, effectiveness or quality claims:

  • Health Canada will not authorize the vaccine

  • the product cannot be sold in Canada

Find out more about Health Canada’s:

  • vaccine approval process

  • review of COVID-19 vaccines

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