Information for Transitioning 21 day to 16 Week 2nd Dose Vaccine Rebooking

2nd Dose Schedule Change

For those who received a first dose of the COVID-19 vaccine between March 3-10 and were originally booked for 21 days from that first appointment, please note that these appointments were rescheduled for 16 weeks from your first booking. 

Your new appointment date and time should have been sent to you by email.


Rationale for Adjusting Time Period Between Doses

To increase the number who will benefit from a first dose of a COVID-19 vaccine, Ontario has increased the time between the first and second dose to 16 weeks. This change will allow more people to get vaccinated sooner for the protection of all. This change follows recommendations from the National Advisory Committee on Immunization (NACI) and applies to the Pfizer-BioNTech, Moderna and AstraZeneca/COVISHIELD vaccines.

Evidence shows that the Pfizer-BioNTech and Moderna vaccines are very effective in reducing symptoms, hospitalizations and death for two months after the first dose, including among older people. Research on the new AstraZeneca/COVISHIELD supports the same positive effects with a longer interval between the first and second dose.

The Vaccine Clinical Advisory Group (VCAG) provides recommendations for Ontario’s COVID-19 vaccine program, including the recent change to a 16 week interval between the first and second dose.

This recommendation will be reviewed on a regular basis to make sure it is based on both new science and changes in vaccine supply. For example, if more vaccine is available and more Ontarians receive their first dose sooner, this could result in the second dose being given before 16 weeks.

With that in mind, the VCAG has reviewed the 16 week recommendation for several specific populations where there is limited data available on vaccine efficacy and effectiveness, who are at increased risk of severe outcomes from COVID-19 and who may have a reduced immune response to vaccines because of their underlying condition.

Recommendations have not changed for pregnant individuals and individuals over 80 years of age; both will continue to receive their second dose at 16 weeks.

However, recommendations have changed for the following groups, who will now receive their second dose based on the recommendations from the vaccine manufacturer; 21 days for Pfizer, 28 days for Moderna, and 4-12 weeks for AstraZeneca.

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

  • 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.
  • living in First Nation communities (including on reserve members and remote and isolated communities supported by Operation Remote Immunity).
  • who are transplant recipients, including solid organ transplants and hematopoietic stem cell transplants.
  • with malignant hematologic disorders and non-hematologic malignant solid tumors 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. 

If these exceptions apply to you and you have already received your first dose, please email the TBRHSC at to reschedule your second dose appointment. Proof will not be required but an attestation form will need to be signed.

This change will be closely monitored, including the impact on variants of concern.

By getting more people vaccinated and protected sooner, this new schedule for second doses will reduce the number of hospitalizations and deaths from COVID-19, benefiting everyone.

For more information, download the Ministry of Health's Extension of the Second Dose Interval (PDF).

Last Updated: 08/04/2021