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COVID-19 vaccine safety

Learn about the safety of COVID-19

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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.

Questions about COVID-19 vaccines? Here are some answers.

Vaccination is expected to be an effective way to prevent the spread and reduce the impact of COVID-19. The effectiveness and immune response of the vaccine is being monitored as the vaccine is rolled out.

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 four vaccines for use in Canada:

  • Pfizer-BioNTech – expected to be 95% effective after two doses

  • Moderna – expected to be 94% effective after two doses

  • AstraZeneca (AstraZeneca and COVISHIELD) – expected to be 64% effective after two doses

The vaccines:

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

  • have met all the requirements for approval, including safety

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

Learn more about the vaccines from Health Canada’s website.

Get vaccine facts and vaccine safety information in multiple languages.

Possible side effects

Like any medication, vaccines can cause mild side effects and reactions. These can last a few hours or a couple of days after vaccination.

Common side effects may include:

  • redness, soreness or swelling on the arm where you got the shot

  • tiredness

  • headache

  • muscle and joint pain

  • chills

  • mild fever

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. Available in described video Get answers to questions about the COVID-19 vaccines from Ontario’s medical professionals

When to call your doctor

Vaccine reactions are rare. However, they may occur up to three days after getting vaccinated.

Call your doctor or health care practitioner if you develop any of the following reactions within three days of receiving the vaccine:

  • hives

  • swelling of the face or mouth

  • trouble breathing

  • very pale colour and serious drowsiness

  • high fever (over 40°C)

  • convulsions or seizures

  • other serious symptoms, such as numbness

If you think you might be having a severe allergic reaction after leaving the vaccination site, call 911.

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

Who should not get the vaccine

You should not get the vaccine if you have:

  • symptoms of COVID-19

  • another acute disease, such as strep throat, flu or pneumonia

Speak with your doctor or health care provider before booking an appointment if you or the person you are booking for:

  • is pregnant or breastfeeding

  • has an autoimmune condition

  • is immunocompromised due to disease or treatment

  • has had severe allergic reactions to vaccinations before

Read the Vaccination Recommendations for Special Populations for more information.

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

Available in described video Get answers to questions about the COVID-19 vaccines from Ontario’s medical professionals

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-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

The AstraZeneca and Janssen vaccines that Health Canada approved are viral vector-based.

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

Vaccine development

Creating a new vaccine typically takes years. However, the COVID-19 vaccines were developed quickly for many reasons, including:

  • being informed by decades of research on other strains of coronavirus prior to COVID-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 makes sure vaccines remain safe for Ontarians by:

  • 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

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 reactions that were not previously reported.

Read more information on vaccines and vaccine authorization updates from the Government of Canada.

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

  • 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-19covid 19 vaccines


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