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Vaccine Safety:
Are Vaccines Safe?

All vaccines are thoroughly tested to assess their benefits and risks before being made available for public use.

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How are vaccines made and tested?


Making a vaccine starts in a lab. Once lab tests show that it can work, it is sometimes tested in animals. If data indicates it may be safe, it is studied to assess the safety profile in people. Then clinical trials with human volunteers may start.



There are 3 phases of clinical trials an investigational vaccine must go through to assess that its benefits are greater than its risks (side effects).

Erlenmeyer flask
Medical staff reviewing notes

Phase 1

The focus of this phase is to understand an investigational vaccine's safety. In this phase, scientists study how the size of a dose may relate to side effects.

Phase 1 clinical trials involve, on average, 20–100 healthy volunteers.
Medical doctor giving lecture

Phase 2

During this phase, scientists perform additional studies to gather more information on side effects. They also study how the size of a dose may relate to an investigational vaccine's ability to work correctly.

Phase 2 clinical trials have several hundred volunteers.
Medical doctor reviewing notes with patient

Phase 3

People who receive an investigational vaccine are compared with people who may receive a placebo or another vaccine. This is done so scientists can learn more about how well the investigational vaccine works and see if there are any side effects. For example, in the case of the global COVID-19 health crisis, participants who received a placebo were also able to receive the investigational vaccine if they chose to participate in the crossover part of the trial.

Phase 3 clinical trials can have a few hundred to thousands of volunteers.

and Distribution

After the local health agency authorizes or approves the investigational vaccine, it can be available for public use.

In certain instances where conditions from a disease could severely affect a large population of people, the agency may issue an Interim Order (IO). An IO helps to get a treatment or investigational vaccine immediately to the people who need it.

Novavax shipping boxes


Federal agencies will continue to monitor an investigational vaccine's safety and watch for possible side effects. High safety standards and testing help to ensure that vaccines in Canada are kept safe.

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Possible Side Effects
From Vaccines

While it is unlikely, severe side effects can occur from investigational vaccines authorized or approved by the FDA. You should talk to your doctor about what to look out for. These side effects can be different for everyone. The most common side effects from vaccines are usually mild and go away quickly on their own.

Some of the common side effects include:

  • Pain, swelling, or redness where the shot was given
  • Mild fever
  • Chills
  • Feeling tired
  • Headache
  • Muscle and joint aches
Person being protected by virus

Side effects from vaccines are usually a sign that your body is starting to build immunity. They typically go away a few days after receiving a vaccine.