Skip to content

UPDATE: Second Dose of the AstraZeneca COVID-19 vaccine in Ontario

  • by

Dear patients,

Today, Ontario’s Chief Medical Officer of Health announced that people who received their first dose of the AstraZeneca (AZ) vaccine between March 10 and March 19 may choose to get their second doses of the vaccine starting Monday, May 24 and up until May 31, 2021.

There is still a pause on the AZ vaccine as a first dose, announced on May 11.
Here is more information, including options right now, for those of you who received a first dose of the AZ vaccine.

What are my options for my second dose if I received one dose of AZ?

  1. If you received your first dose of AZ between March 10 and 19, you may choose to get vaccinated during the week of May 24 through 31 with a shortened 10-week interval between doses. If you do not wish a shortened dose interval, you can wait to receive a second AZ dose at the 12-week interval. For those wishing the earlier second dose at the 10-week interval, please contact the location where you received your first dose to book an appointment for your second dose.

  2. If proven to be safe and effective, you may choose to wait to have a second dose with one of the mRNA vaccines (Pfizer or Moderna) at a 16-week interval (see more information below on what we know about the evidence for mixing and matching vaccines).

  3. For anyone vaccinated from March 20 onwards, or those vaccinated between March 10-19 who wish to have a 12-week interval between AZ doses, please stay tuned. Further details will be provided soon by the Ministry of Health on booking second doses.

Please review this consent form in the meantime for more information on options for a second dose. You can also call us to discuss the benefits and risks if you have additional questions.

What is the best interval between getting my AZ shots?

Clinical trials showed that AZ works best when the two doses are spread out by at least 12 weeks – estimated at over 80% protection against symptomatic disease versus an estimated 69% when given between nine and 12 weeks. Your personal circumstances, including COVID-19 prevalence in your community and your risk of being infected by the people around you, should be a consideration in your decision.

Is it safe to get the AZ vaccine as a second dose?

  • The AZ vaccine is approved by Health Canada.

  • We know that a second dose provides greater and significant protection than one dose alone against serious illness, hospitalization, and death from COVID-19.


The risk of a serious blood clot (VITT) associated with the second dose of the AZ vaccine is currently estimated at approximately 1 in 600,000, based on data from the United Kingdom. If/when you receive the AZ vaccine, know the signs of the associated rare blood clots because it can be best treated if addressed early. Please reach out to our office or seek emergency care if you have the warning symptoms. [For a quick summary of symptoms, see graphics here:]

Is it safe to get a different brand of vaccine as my second dose?

There is no published data so far on effectiveness of mixing and matching different vaccine brands, but a large clinical trial is underway with results anticipated in June 2021. The early data looking at side effects in the first few days show that mixing doses results in more significant side effects but not severe enough to land people in hospital.

Canada’s National Advisory Committee on Immunization is expected to make recommendations based on these findings in the upcoming weeks.

Where can I get updates?

We will update you as soon as we have information. You can also check the provincial and our local public health unit website.

Please remember – our office is open to you, by phone, by video or in person to answer any vaccination questions. We are here to help keep you safe and get through this pandemic together.


The Doctors at HS Medical


Additional Resources: