How vaccines are tested, approved, and monitored in Canada, what each ingredient category actually does, and how safety is tracked over time — explained clearly, without minimizing real questions.
Before any vaccine can be used in Canada, it goes through a long process of testing and independent review. No single person or company decides on their own that a vaccine is ready for use.
Laboratory and early research
Vaccines begin with years of laboratory research to understand how they work and to check for early signs of safety and effectiveness.
Clinical trials in people
Vaccines are then tested in carefully designed clinical trials that move through phases, starting with small groups and expanding to thousands of volunteers, measuring both safety and how well the vaccine works.
Health Canada review and authorization
Health Canada independently reviews all the evidence — including detailed manufacturing and quality information — before deciding whether to authorize a vaccine for use. This is a rigorous, science-based review.
NACI and Ontario program decisions
The National Advisory Committee on Immunization (NACI) reviews the evidence and makes recommendations on how vaccines should be used. Ontario then decides which vaccines to include in its publicly funded program.
Approval isn't the end of the process. Once a vaccine is in use, its safety continues to be tracked closely — a system often called post-market surveillance. Because far more people receive a vaccine after approval than during trials, this ongoing monitoring can detect very rare events that trials might not.
How rare side effects get detected
If a vaccine could cause a side effect that happens in, say, one in a million people, a trial of a few thousand people might never see it. Ongoing monitoring across millions of doses is how these very rare events are identified, studied, and acted on if needed. This is a strength of the system, not a weakness — it means safety review never really stops.
What "AEFI" means
You may encounter the term AEFI, which stands for "adverse event following immunization." This is an important and often misunderstood term: it means any health problem that happens after a vaccine — but "after" does not mean "caused by." Many AEFIs turn out to be coincidental and unrelated to the vaccine. Reporting them anyway is what allows experts to look for genuine patterns.
In Ontario, healthcare providers report AEFIs to their local public health unit, which feeds into provincial and national safety surveillance systems. This is how potential safety signals are identified and investigated. If your child has a notable reaction after a vaccine, telling your healthcare provider not only helps your child — it also contributes to this safety system.
What clinics do to prepare for reactions
Severe allergic reactions to vaccines are rare, but clinics prepare for them. Staff are trained to recognize and treat allergic reactions, and clinics keep epinephrine on hand. This is also why a short observation period — often around 15 minutes — is recommended after vaccination: serious allergic reactions, when they do occur, typically happen soon after the injection, when help is right there.
Vaccine ingredients can sound unfamiliar, but each one has a specific job. Importantly, the amount matters — many ingredients people worry about are present in tiny quantities, often smaller than what we encounter in everyday food and life. Below are the main categories. For exact, product-specific ingredients, each vaccine profile includes a table sourced from the official Health Canada product monograph.
Active ingredients (antigens)
The part that actually trains the immune system — a weakened or inactivated germ, or a piece of one (like a protein or sugar). This is what teaches the body to recognize and fight the real disease.
Adjuvants
Substances (such as aluminum salts) that help the immune system respond more strongly, so the vaccine works better — sometimes allowing smaller amounts of antigen or fewer doses. Aluminum adjuvants have a long safety record and are used in small amounts.
Stabilizers
Ingredients (such as sugars, amino acids, or gelatin in some products) that keep the vaccine stable and effective during storage and transport — for example, protecting it from heat or freeze-drying.
Preservatives
Used in some vaccines (especially multi-dose vials) to prevent contamination. Many modern single-dose vaccines contain no preservative at all.
Buffers & tonicity agents
Ingredients like salts and acids/bases that keep the vaccine at the right acidity and concentration so it's gentle on the body's tissues. Ordinary salt (sodium chloride) is a common example.
Manufacturing residuals
Tiny traces left over from how the vaccine is made — such as small amounts of antibiotics, egg or yeast proteins, or substances used to inactivate a germ. These are present in trace amounts and are relevant mainly for people with specific severe allergies.
Why "the dose makes the difference"
Some ingredients that sound alarming are actually substances the body encounters or even produces naturally. For example, formaldehyde is used to inactivate certain germs during manufacturing, and only trace amounts remain — less than the body naturally produces and carries in the blood as part of normal metabolism. The presence of an ingredient matters far less than the amount, and these amounts are very small and reviewed for safety.
Why product monographs matter
Every authorized vaccine has an official product monograph — a detailed document reviewed by Health Canada that lists its exact ingredients, how it's made, its known side effects, and its precautions. This is the authoritative source for what's actually in a specific product, and it's what we use for the ingredient tables in our vaccine profiles. You can look these up yourself through the Health Canada Drug Product Database (linked on our Resources page).
If your child has significant allergies, has had a serious reaction to a previous vaccine, or has a known allergy to a specific vaccine component, this is worth discussing with a healthcare provider before the appointment. They can review the specific ingredients (using the product monograph), advise on any precautions, and in some cases recommend that vaccination happen in a setting with extra monitoring.
Having an allergy to something like eggs or a particular antibiotic doesn't automatically mean a child can't be vaccinated — it depends on the specific vaccine and the specific allergy. The key is to have the conversation, so the right plan can be made for your child. Each vaccine profile includes a "Who should pause or check first" section with vaccine-specific guidance.
Sources
Please note: This is an independent website. It is not affiliated with, endorsed by, or operated by the Government of Ontario, the Ministry of Health, or any public health unit. The information here is for general education only and is not medical advice — always consult a licensed healthcare provider about your or your child's immunizations.