Ontario won’t be offering AstraZeneca’s COVID-19 vaccine as a first dose option anymore because of concerns over blood clots. 

Chief Medical Officer Dr. David Williams said the decision was made out of an abundance of caution after an increase in cases of rare blood clots associated with the vaccine, known as Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT).

Canada has seen 12 cases of the blood clots with three patients dying. There have been eight cases in Ontario. 

In Ontario as of May 8th, 651,012 doses of the vaccine had been given with an incidence rate of VITT of 0.9 for every 100,000 shots. 

But in the last few days that has increased to 1.7 per 100,000 doses given, or about 1 in 60,000.

Dr. Williams said new data from the U.K. shows the risk of developing the blood clot after a second AstraZeneca dose goes down significantly if the person had no VITT side effects after the first dose, to about one in a million.

Health officials are also reviewing data as to whether to give a second dose of the AstraZeneca vaccine.

He said all levels of government are currently looking at offering Pfizer or Moderna vaccines as a second dose for anyone who did receive an AstraZeneca shot and are seeking input from the National Advisory Committee on Immunization on whether “vaccine mixing” is suggested. 

Williams also cited a reliable and steady supply of the Pfizer and Moderna vaccines as a reason for the pause.