Ontario will proceed to administer second doses of the AstraZeneca COVID-19 vaccine, beginning with those who received their first dose between March 10 and 19 — when the AstraZeneca vaccine first became available in the province.
“Nothing is more important than the health of Ontarians, and for the best protection against COVID-19 it is vital that everyone who received the AstraZeneca vaccine for their first dose receives a second dose of a COVID-19 vaccine,” said Dr. David Williams, Ontario’s chief medical officer of health, during a virtual media briefing on Friday (May 21) with Dr. Dirk Huyer, coordinator of the provincial outbreak response.
Beginning the week of May 24, for a limited time, people who received their first dose of AstraZeneca from March 10 and 19 can contact the pharmacy or primary care provider where they received their first dose and book an appointment for their second dose.
The province announced on May 11 it was pausing the administration of first doses of AstraZeneca, because of an observed increase in Ontario of an adverse reaction known as vaccine-induced immune thrombotic thrombocytopenia (VITT). The risk of VITT in Ontario was observed to be 1 in every 60,000 people vaccinated rather than the previously observed rate of 1 in every 100,000 vaccinated.
While the pause on first doses is still in effect while the province’s health experts review data on the VITT risk for first doses, Dr. Williams said data from the UK suggests the risk of VITT in people receiving their second dose of AstraZeneca is one in 600,000 — higher than the previously observed rate of one in one million but still relatively rare.
The only people who received their first dose of AstraZeneca from March 10 and 19 were Ontarians aged 60 to 64 who booked an appointment at one of 325 pharmacies in Toronto, Windsor-Essex, and Kingston, Frontenac, Lennox & Addington, or were contacted by primary care providers in Hamilton, Toronto, Guelph, Peterborough, Simcoe-Muskoka, and Peel.
People vaccinated during this period who consent to receive the second dose, will be receiving it at 10 to 11 weeks instead of the original interval of 16 weeks. The original clinical data for the AstraZeneca vaccine indicates the second dose should be administered no later than 12 weeks after the first dose, and studies from the UK and other jurisdictions have shown the highest efficacy of the vaccine occurs when the second dose is administered at 12 weeks.
“We are providing these individuals with a safe option based on the available data and evidence and will continue to work with Health Canada to monitor the quality and efficacy of all COVID-19 vaccines being used in Ontario,” Dr. Williams said.
Ontario has 55,000 doses of AstraZeneca that will expire by May 31. Dr. Huyer said more than 55,000 people received their first dose between March 10 and 19, so not everyone who received their first dose during this period will be able to get a second dose.
Dr. Williams said people who received their first dose of AstraZeneca after March 19, as well as for those who received it between March 10 and 19 but do not get a vaccine appointment during the week of May 24, will be able to receive their second doses within the now-recommended interval of 12 weeks.
The provincial government is working with primary care providers and pharmacies to ensure second dose appointments are scheduled in advance of the 12-week interval, and will provide further information in the near future on how people who received their first dose of AstraZeneca can book a second dose appointment.
As for the possibility of people who received their first dose of AstraZeneca receiving a second dose of Pfizer or Moderna instead, Dr. Williams said the National Advisory Committee on Immunization (NACI) is currently not recommending vaccines of different types (e.g., mRNA vaccine and viral vector vaccine) be mixed, since the efficacy of mixing vaccine types is not yet known.
That may change next month however, as Dr. Williams said NACI is reviewing evidence from a clinical trial in the UK evaluating the administration of two doses of different vaccines and expects NACI will be making a recommendation in June.
