Risk of blood clots tied to AstraZeneca shot now estimated at 1 in 55,000 in Canada The future use of the AstraZeneca-Oxford COVID-19 vaccine in Canada is now in question due to concerns over the increased risk of rare but severe blood clots connected to the shot, an unpredictable future supply and a significant amount of other vaccines. Alberta was the first province to confirm it would stop administering first doses of the AstraZeneca vaccine, citing a scarcity of supply. The province will instead prioritize mRNA vaccines like Pfizer-BioNTech and Moderna while reserving existing AstraZeneca for second doses. Ontario’s Chief Medical Officer Dr. David Williams announced in an impromptu news conference late Tuesday afternoon the vaccine would also no longer be offered to Ontarians as a first dose and future supply would instead be reserved for optional second shots. But unlike Alberta, Ontario’s decision was made largely due to the rising rate of the rare but serious blood-clotting condition connected to the shot known as vaccine-induced immune thrombotic thrombocytopenia (VITT).