Flu shot ingredients

Vaccines contain very small amounts of specific ingredients, all of which play necessary roles either in making the vaccine, or in ensuring that the vaccine is safe and effective.


Thimerosal is a preservative used in multi-dose vials of the flu vaccine to keep them free from contamination and contains an organic form of mercury called ethyl mercury.

Ethyl mercury is not the same as methyl mercury, which may be found in fish and shellfish. Methyl mercury stays in the body and can make people sick, while thimerosal doesn't stay in the body. There is no evidence of harm caused by the low doses of thimerosal in vaccines.

Flu vaccines available as single-dose, pre-filled syringes do not contain thimerosal.


Formaldehyde occurs naturally in the human body and helps us digest food.

It is used to inactivate viruses and bacteria that might contaminate the vaccine during production. Almost all of the formaldehyde is removed before the vaccine is packaged. There is approximately 10 times more formaldehyde in a baby’s body at any time than there is in a flu vaccine.


All free flu vaccines provided by Ontario are aluminum-free. Aluminum is used in some other types of vaccines to enhance the immune system’s response after immunization.

Egg allergies

The flu vaccine is safe for people with egg allergies.

Guillain-Barre Syndrome

You are more likely to get Guillain-Barre Syndrome from the flu than from the flu shot. The chance of getting this rare illness from the flu shot is one in a million.

The syndrome causes your body’s immune system to attack your nerves – weakness and tingling in your hands and feet are usually the first symptoms.

Contact your doctor or nurse practitioner if you experience any of these symptoms after getting a flu vaccine or when you have the flu.

Flu shot effectiveness

How well the flu shot works can vary from year to year. When the vaccine matches the flu strains in a particular flu season well, it can prevent the flu in up to 60% of the population.

For high-risk adults, including seniors, studies have shown that flu shots decrease the number of:

  • doctor visits
  • hospital admissions
  • deaths

The flu shot will not protect against COVID-19 and other coronaviruses such as the common cold. Take actions to help stop the spread of COVID-19.

Vaccine safety

The flu vaccines approved for use in Canada are safe.

Canada has a strong system in place to monitor the safety of vaccines. Health Canada approves and monitors all vaccines. Every single batch of vaccine is tested according to strict guidelines for safety and quality before it is used. Once it is in use, there are several systems in place to monitor the vaccine’s safety.

Vaccine manufacturers and providers must report adverse events from vaccines.

Public health units in Ontario investigate all reports of adverse reactions following immunization, and these events are then reported to the provincial and national levels to support ongoing vaccine safety surveillance and monitoring efforts. The safety of vaccines available in Canada is continually monitored through this process.

Learn more about adverse events and your role in keeping vaccines safe (PDF).

When to avoid the flu shot

You should not get a flu shot if you have:

  • have serious allergies (anaphylaxis) to any ingredient in the vaccine, except eggs. Your health care provider can tell you which ingredients are in the vaccine and if it is safe for you
  • have experienced a serious allergic reaction from a previous flu shot
  • developed Guillain-Barre Syndrome within six weeks of a previous flu shot; contact your primary health care provider to discuss and learn more

You should wait to get your flu shot if you have:

  • have a severe acute illness with or without fever
  • think you have COVID-19 symptoms , such as acute respiratory infection, sore throat,  nausea, vomiting or diarrhea
  • have a probable or confirmed case of COVID-19
  • are in close contact of someone who has COVID-19

If you have a severe acute illness, you should contact your health care provider to determine if you should get a shot.

If you have a minor illness, with or without a fever (for example, a cold), you can still get the flu shot.

If you have symptoms of acute respiratory infection, including minor symptoms such as sore throat, you should:

If you test negative and pass the self-assessment, you may get the flu shot. 

If you are self-isolating during COVID-19, (either as a suspected, probable or confirmed COVID-19, or due to exposures), you should not attend a flu shot appointment or clinic during your period of isolation as you pose an unnecessary risk to the public and health care providers.