Ontario pauses first doses of AstraZeneca vaccine

Dr. David Williams, Ontario's chief medical officer of health discusses pausing first doses of the AstraZeneca vaccine. May 11, 2021. (Capture via Ontario.ca)

Ontario Health says it has stopped administering first doses of the AstraZeneca COVID-19 vaccine over concerns about rare blood clots.

During a news conference Tuesday, Ontario Chief Medical Officer of Health Dr. David Williams said, over the last several days, the province has reported a growing number of vaccine-induced immune thrombotic thrombocytopenia (VITT) incidents.

“The decision to pause [the AstraZeneca vaccine] is also based on the increased and reliable supply of the Pfizer and Moderna mRNA vaccines, and also a recent downward trend of cases,” Dr. Williams said.

As of May 8, over 650,000 doses of the AstraZeneca vaccine have been administered with a VITT rate of 0.9 per 100,000 doses. Over the last few days, Dr. Williams says the number has increased to 1.7 per 100,000 doses administered.

Dr. Jessica Hopkins, the province’s chief health protection and emergency preparedness officer, said there had been eight cases of VITT reported in Ontario as of May 8. The rate of having a side effect has gone from one in 100,000 to one in 60,000.

“When we look at VITT there’s a spectrum of illness that can happen from people who are mildly symptomatic to people who can become severely ill with the disease,” Dr. Hopkins said. “When you look at the risk of COVID disease, it’s very much based on your individual situation.”

Dr. Hopkins added that, with the numbers of COVID-19 going down in Ontario and an increased safety concern, it makes sense to pause initial doses of the AstraZeneca vaccine due to the severe outcomes associated with VITT.

Following the decision, Dr. Williams said the province will be reviewing alternative options for those who have already received their first dose of the vaccine. He said the province is not certain as to when or if more AstraZeneca doses will arrive. Ontario has 49,280 doses on hand out of over 707,000 received which will be reserved for second doses.

Health officials say the chance for a VITT event to occur is lower with the second dose of the vaccine according to data from the United Kingdom. Dr. Williams said they will be reviewing the data in order to provide further information to the public.

Ontario has called on the National Advisory Committee on Immunization (NACI) to provide  direction on mixing the Pfizer and Moderna mRNA vaccines for first and second doses.