COVID-19 Information on Vaccine Second Doses

The Province of Ontario has announced the accelerated rollout of second doses for COVID-19 vaccines. Eligibility has expanded to allow Ontarians to receive the vaccine ahead of schedule, providing a strong level of protection against the virus.

Please note: Individuals have the option to keep their original second dose appointment but are encouraged to get their second dose as soon as they are able. Appointments will not be automatically canceled or rebooked.

If you choose to rebook to an earlier date, visit the COVID-19 Vaccine Appointments page for booking options. You also need to cancel your original appointment. Instructions to cancel if you had a second dose appointment:

  • Provincial Booking System - use the link to cancel/reschedule in the appointment confirmation email that you received when you first booked or you can use the Manage your Appointment(s) page.
    • NOTE: As of June 14, 2021, pre-cancellation notifications will be emailed out to anyone who has received both doses of COVID-19 vaccine and still has an existing appointment in the provincial booking system. Another notification will be sent in 4 days once the appointment has been cancelled.
  • TBDHU COVID-19 Vaccine Appointment Booking System - use the link to cancel in the appointment confirmation email that you received when you first booked or you can use the Cancel Appointment page.

If you had a second dose at any other clinic/location, please contact them directly.


Groups Eligible for an Earlier Second Dose


  • Youth aged 12-17 (or turning 12 in 2021) who have received their first dose of Pfizer. Second dose appointment must be at least 28 days following the first dose.
  • Individuals who received their first dose of an mRNA vaccine (Pfizer or Moderna). Second dose appointment must be at least 28 days following the first dose.
  • Individuals who have received a first dose with AstraZeneca vaccine at least 8 weeks before the second dose appointment date. See the AstraZeneca section below for more details.

Other Groups

There are some groups who remain at the dose interval outlined by the vaccine product monographs. See the list below of groups eligible for the original second dose interval


Interchangeability of mRNA vaccines (Pfizer and Moderna)

Individuals now have more flexibility to receive different mRNA vaccines for their first and second doses. This will support individuals in getting fully immunized more quickly and conveniently by giving them more options. This will also help offset issues related to vaccine supply and delivery.

The National Advisory Committee on Immunization (NACI) and Ontario Ministry of Health have recently confirmed that a mixed vaccine schedule for the COVID-19 vaccine is safe, effective, and will complete the vaccine series. This means that individuals who received a first dose of an mRNA vaccine (Pfizer or Moderna) can receive a different mRNA vaccine for their second dose.

The option of mixing of vaccines for individuals to complete the two dose series is reasonable given the similarity of these vaccines in their mechanism of action, side effect profile and high effectiveness against COVID-19 and its variants, including the Delta variant. At this time, the mixed schedule does not apply to youth 12 to 17 years for whom only Pfizer is approved.

Individuals are encouraged to get their second dose as soon as they are able, and to get the vaccine available at the clinic most convenient for them, regardless of which mRNA vaccine that it is offered at that clinic. TBDHU will provide information on what vaccine is available at clinics; however, vaccine availability may change depending on vaccine supply and delivery schedules.

Read more in the Ministry of Health's A Safe and Effective Second Dose (PDF) resource.


Individuals who Received the AstraZeneca Vaccine for their First Dose

As of June 12, 2021, Ontario accelerated the second dose interval for those who received a first dose of AstraZeneca to eight weeks with informed consent, from 12 weeks. See the media release for details.

An mRNA vaccine is now preferred as the second dose for individuals who received a first dose of the AstraZeneca/COVISHIELD vaccine, based on emerging evidence of a potentially better immune response from this mixed vaccine schedule and to mitigate the potential risk of VITT associated with viral vector vaccines. As of September 3, 2021, AstraZeneca/COVISHIELD is no longer available at local pharmacies.

Current evidence recommends waiting 12 weeks to receive a second dose as this interval allows for a better/optimal immune response to be achieved. However, the province recognizes that some may want to receive their second dose sooner to have the increased protection provided by an earlier second dose.

Those who choose to book an mRNA vaccine (Pfizer-BioNTech or Moderna) appointment should visit the COVID-19 Vaccine Appointments page to find a clinic.

Additional information for those who choose an mRNA vaccine (Pfizer-BioNTech or Moderna) as their second dose:

  • The National Advisory Committee on Immunization (NACI) updated its guidance to recommended that a first dose of the AstraZeneca vaccine can be followed by an mRNA vaccine (Pfizer-BioNTech or Moderna).
  • The updated NACI guidance is based on emerging research from Spain and the United Kingdom that found mixing and matching the AstraZeneca and Pfizer-BioNTech vaccines was both safe and effective at preventing COVID-19.
  • Health Canada is monitoring the situation and reviewing new research from around the world as it becomes available.
  • Individuals with questions should speak to their health care provider about their own specific situation.
  • Individuals will be considered “fully immunized” if they choose an mRNA vaccine (Pfizer-BioNTech or Moderna) for their second dose.


Don't know what vaccine you received for your first dose?

For those who are unsure what COVID-19 vaccine they received for their first dose, they should check their original appointment documentation or their printed/emailed receipt. Receipts can also be accessed through the COVID-19 Vaccination website (not accessible using Internet Explorer):

  • Scroll to the bottom of the page and if you read, understand and accept the terms of use, click “Continue”.
  • Complete the Identity Verification Information and click “Continue”.
  • Click “Continue” in the yellow box titled “Vaccination Receipts”.
  • You will now be able to review your results and access a PDF receipt.



Groups Eligible for Original Second Dose Interval

The dose interval will remain as per the product monograph for the following groups:

  • Those living in long-term care and retirement homes, Elder Care Lodges and Assisted Living facilities who are at the greatest risk of serious illness and death.
  • Those living in First Nation communities (including on reserve members and remote and isolated communities supported by Operation Remote Immunity).
  • Those who are transplant recipients, including solid organ transplants and hematopoietic stem cell transplants.
  • Those with malignant hematologic disorders and non-hematologic malignant solid tumours receiving active treatment, (chemotherapy, targeted therapies, immunotherapy), excluding individuals receiving solely hormonal therapy or radiation therapy. They should get the vaccine at a time when their immune system is best able to respond; this would happen in consultation with their health care provider. 
  • Those who are undergoing hemodialysis or peritoneal dialysis in the authorized age groups (eligible individuals under this category will be contacted by the TBRHSC directly).
  • Those who are urban Indigenous peoples and their family circle (eligible individuals should receive their second dose through the organization/clinic where they received their first dose. See the "Appointments for Indigenous Individuals" tab on the COVID-19 Vaccine Appointments page for links to urban Indigenous clinics in Thunder Bay).
  • Those who are high-risk health care workers. For a list of eligible workers, see the Ministry of Health backgrounder: High-Risk Health Care Workers Eligible to Receive a Second Dose of the COVID-19 Vaccine at a Shortened Interval. TBRHSC will contact eligible individuals to reschedule their second dose appointments.
  • Those in the authorized age groups who are taking an anti-CD20 agent (e.g. rituximab, ocrelizumab, ofatumumab). The timing of the vaccine should be decided with the treating provider in order to optimize the immune response from the vaccine series and minimize delays in management of their underlying condition.
  • Individuals for whom transplant or dialysis initiation is imminent, or whose cancer treatment (excluding solely radiation or hormonal therapy) or anti-CD20 agent therapy, is to be imminently initiated.

Individuals in the above groups can visit the TBRHSC COVID-19 Vaccinations page for information or email questions to

Last Updated: 10/09/2021