Ontario paused the use of the AstraZeneca vaccine as a first dose against COVID-19 Tuesday due to increased concern over blood clots. Ontarians who have already received a first dose have yet to be told whether they should switch to a different vaccine for the second shot or even if they’ll have the option of sticking with AstraZeneca. There have been eight cases in the province of vaccine-induced immune thrombotic thrombocytopenia (VITT), a still rare but potentially serious side effect of the vaccine. Chief Medical Officer of Health Dr. David Williams said the decision was made out of an abundance of caution as new data suggested the risk of clots was higher than first thought. “We know that those that made a decision before to get it — it was a good decision,” Williams said.