Additional Dose Information

Full vaccination offers the best protection against serious illness, hospitalization, and death due to COVID-19. The greatest risk for COVID-19, serious illness, and death remains in those who are not yet vaccinated.

Second dose

The Ministry of Health is recommending an interval of eight weeks between first and second doses. There is emerging evidence that longer intervals between the first and second doses of COVID-19 vaccines result in more robust and durable immune response and higher vaccine effectiveness.

If you received Moderna, Pfizer or AstraZeneca as your first dose, you may receive a second dose of Pfizer or Moderna eight weeks (56 days) after your first dose, or as directed by your health care provider.

Please note, in Ontario the AstraZeneca vaccine is not available for first or second doses.

A complete two-dose COVID-19 vaccine series provides strong protection against COVID-19 infection and severe outcomes, including against the Delta variant, in the general public.

Mixing vaccine types for the second dose

If you received an mRNA vaccine (Pfizer or Moderna) for your first dose, you can safely take either Moderna or Pfizer for your second dose for strong protection. Both Moderna and Pfizer vaccines are authorized for use in Canada and use a similar mRNA technology, so the vaccines are safe to mix. This is consistent with recommendations provided by the National Advisory Committee on Immunization and with practices within several provinces.

Third or booster dose

In Ontario, select vulnerable populations are eligible for a third dose. Read the Ontario Ministry of Health COVID-19 Third Dose Recommendations.

A booster dose may be administered at a minimum of three months (84 days) after the second dose for:

  • All Ontarians 18 years of age and older
  • Non-Indigenous adults in First Nations, Métis and Inuit households over the age of 18
  • Those from First Nations, Métis and Inuit populations who are 16 years
  • Health care workers who are 16 years of age or older
  • Designated essential caregivers in congregate settings
  • Anyone who received a complete vaccine series with a viral vector vaccine (either two doses of the AstraZeneca vaccine or one dose of the Janssen vaccine)
  • Residents of high-risk congregate settings including long-term care homes, higher-risk licensed retirement homes and First Nations elder care lodges, and elderly living in other congregate settings (e.g. assisted-living facilities, chronic care hospitals, naturally occurring congregate retirement settings/congregate senior’s apartment buildings)
A third dose may be administered at a minimum of eight weeks (56 days) after the second dose for individuals 5 years of age or older with the following qualifying medical conditions:
  • Individuals receiving active treatment (e.g., chemotherapy, targeted therapies, immunotherapy) for solid tumour or hematologic malignancies
  • Recipients of solid-organ transplant and taking immunosuppressive therapy
  • Recipients of chimeric antigen receptor (CAR)-T-cell therapy or hematopoietic stem cell transplant (within two years of transplantation or taking immunosuppression therapy)
  • Individuals with moderate to severe primary immunodeficiency (e.g., DiGeorge syndrome, Wiskott-Aldrich syndrome)
  • Individuals with stage 3 or advanced untreated HIV infection and those with acquired immunodeficiency syndrome
  • Individuals receiving active treatment with the following categories of immunosuppressive therapies: anti-B cell therapies (monoclonal antibodies targeting CD19, CD20 and CD22), high-dose systemic corticosteroids, alkylating agents, antimetabolites, or tumour-necrosis factor (TNF) inhibitors and other biologic agents that are significantly immunosuppressive.

These individuals are also eligible for a booster dose (fourth dose) 84 days after their third dose. 

Individuals who have any of the qualifying medical conditions and are taking medications listed in the Province's COVID-19 Vaccine Third Dose Recommendations should bring their prescription with them when they attend a clinic as verification of eligibility.

Those individuals who have any of the qualifying medical conditions but are not taking medications listed in the Province's COVID-19 Vaccine Third Dose Recommendations need to have their physician or specialist complete a referral form which they must present at their vaccination appointment.

Fourth Dose

Residents who are severely to moderately immunocompromised can book an appointment for a fourth dose of the COVID-19 vaccine as long as it has been 84 days since their third dose. A fourth dose of an mRNA vaccine will offer more protection to those who are most at risk of serious outcomes from COVID-19 and the Omicron variant.

Regional vaccination clinics, participating pharmacies, and doctors’ offices (for their patients only) will be able to administer these booster doses. Residents who are eligible for a fourth (or any other) dose of the vaccine can book an appointment for a vaccine at a regional vaccination clinic on the Region of Waterloo website.

Some regional vaccination clinics will also be accepting walk-ins, as capacity allows.

Residents who have questions about their health condition and getting a fourth dose of the vaccine should speak with their medical specialist or primary care provider.

Mixing vaccine types for the third or booster dose

If you received an mRNA vaccine (Pfizer or Moderna) for your second dose, you can safely take either Moderna or Pfizer for your third dose for strong protection. Both Moderna and Pfizer vaccines are authorized for use in Canada and use a similar mRNA technology, so the vaccines are safe to mix. This is consistent with recommendations provided by the National Advisory Committee on Immunization and with practices within several provinces.

If you received AstraZeneca for your second dose, a booster dose of an mRNA vaccine (Pfizer or Moderna) is recommended.

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