Dr. David Williams, Ontario's chief medical officer of health, announced Tuesday that the province will stop administering first doses of AstraZeneca COVID-19 vaccine due to increased concerns around vaccine-induced immune thrombotic thrombocytopenia (VITT), which causes blood clots.
"This decision was made out of an abundance of caution due to an observed increase in the observed blood clotting condition...linked to AstraZeneca vaccine," Dr. Williams said.
Ontario's chief medical officer of health confirmed that as of May 8, 651,012 doses of the AstraZeneca vaccine were administered with a rate of VITT of 0.9 per 100,000 dose administered. A total of 202,873 doses of the COVISHIELD vaccine were administered with a rate of VITT of one per 100,000 doses.
"Over the last few days, there has been increase reports of VITT, in particular related to the AstraZeneca vaccine, with a rate of 1.7 per 100,000 doses administered," Dr. Williams said.
Dr. Jessica Hopkins, chief health protection and emergency preparedness officer at Public Health Ontario, confirmed that the province is reporting that the risk of VITT is now one in 60,000 in Ontario. The risk nationally is one in 55,000.
"At a population level, it makes sense to pause AstraZeneca because the risk of severe outcomes with VITT shouldn’t be underestimated," Dr. Hopkins said.
"At the individual level,...there are individual decisions that can be made as well and those are much more nuanced. They depend on your own health, your own risk factors and all of those other things."
Dr. Dirk Huyer, coordinator of the provincial COVID-19 outbreak response, stressed that while this has been considered a rare event, the adverse effect is "severe."
What if you already took the first dose of AstraZeneca?
Despite these concerns, Ontario's chief medical officer of health said Ontarians who received their first dose of the AstraZeneca vaccine made the right decision.
"We maintain that those who received their first dose with the AstraZeneca vaccine did absolutely the right thing to protect illness, and to protect their families, loved one and communities," Dr. Williams said.
"Part of our role with our vaccine safety program,...we're looking for every possibility, every inch of issues that [have] happened, and then we're moving on an abundance of caution to say, let's pause and asses this carefully."
How will people who received their first dose of AstraZeneca COVID-19 vaccine in Ontario get their second dose?
Ontario's chief medical officer of health said he hopes to have answers in the coming weeks around what the options will be for people in the province who are waiting for their second dose, after receiving the first shot of AstraZeneca, stating that the data is being reviewed.
The province has also asked the National Advisory Committee on Immunization (NACI) to provide direction related to the interchangeability of vaccine doses, meaning getting Pfizer or Moderna for their second dose.
Dr. Williams highlight that data from the U.K. is showing there is a reduced risk of VITT when individuals get their second dose, about one in a million, and there are "promising results" of mixing vaccine doses.
Dr. Huyer added that there will also be considerations made around the expiry of currently supply and upcoming shipments, in terms of future administration of AstraZeneca COVID-19 doses.
"This is still a very rare side effect," Dr. Hopkins stressed. "The reason that we're talking about it is because it was one in 100,000 before."
Health experts in Ontario have taken to social media to respond to this latest development in Ontario.
Other individuals also shared reactions on social media in response to the news out of Ontario.