Ontario to start administering second doses of AstraZeneca COVID-19 vaccine next week

The Ontario government announced Friday morning that the province will begin to administer second doses of AstraZeneca COVID-19 vaccines next week.

The week of May 24, anyone who received their first dose between Mar. 10 and Mar. 19 are eligible for their next dose, receiving their second shot at the 10-week interval.

In that March timeframe, AstraZeneca dose were being administered to people in Toronto, Kingston and Windsor as part of a pilot project, in addition to some primary care practices in other regions, including Peel, Hamilton, Wellington Peterborough and Muskoka.

"Nothing is more important than the health of Ontarians and for the best protection against the COVID-19, it is vital that everyone who received the AstraZeneca vaccine for their first dose receives their second dose," Dr. David Williams, Ontario's chief medical officer of health said at a press conference on Friday.

"Choosing to receive the second dose of AstraZeneca vaccine at the 10-week interval is safe and [provides] strong protection against the COVID-19 infection."

Individuals who do not receive their second dose over the week of May 24 will be able to make an appointment at the 12-week interval, by contacting the pharmacy or primary care provider who administered their first shot.

Dr. Dirk Huyer, coordinator of the provincial outbreak response, said that approximately 55,000 doses of AstraZeneca vaccine have a date of effective period of delivery of May 31, which is the focus of this initial rollout.

"You contributed significantly to the population coverage of vaccine, which has had a broad effect on our population in Ontario to increase the percentage of vaccination and therefore decrease the burden of disease," Dr. Huyer said in a message to anyone in Ontario who received the AstraZeneca vaccine.

Dr. Williams said the province is also looking closely at available data and evidence from a clinical trial in the U.K., which has shown early promising results of administering doses of two different vaccines.

The National Advisory Committee on Immunization (NACI) currently does not recommend that vaccine mixing occurs. Dr. Williams identified that this is data on the immune response following the mixed vaccine schedules are not yet available.

"We still have the pause on the first dose because we see the rate of the adverse event, the [vaccine-induced immune thrombotic thrombocytopenia (VITT)], has remained down in the one in 60,000, perhaps going a bit lower as we are still looking for some newly reported cases," Dr. Williams said.

Following this announcement, many Ontarians took to social media to comment on the change: