Canada's most populous province of Ontario on Tuesday suspended use of the AstraZeneca Covid vaccine as a first dose, citing new data showing increased risks of blood clotting.
The move followed the province of Alberta's announcement to also stop administering the vaccine over possible supply shortages.
The two provinces combined represent half the population of Canada.
"Effective today, Ontario will be pausing the rollout and administration of first doses of the AstraZeneca vaccine at this time," Ontario chief medical officer of health David Williams told a news conference.
"This decision was made out of an abundance of caution due to an observed increase in the rare blood clotting condition, known as vaccine-induced immune thrombotic thrombocytopenia, linked to the AstraZeneca vaccine," he said.
Ontario has administered more than 650,000 AstraZeneca doses. Prime Minister Justin Trudeau and his wife Sophie were among the recipients.
In Canada, at least 12 clotting cases have been confirmed out of more than two million administered AstraZeneca doses. Three women have died from clotting.
Earlier, Alberta -- with the highest per capita number of Covid cases of any Canadian province or US state -- altered its immunization strategy, saying it wasn't sure when it would receive additional shipments of the AstraZeneca vaccine.
"Based on global supply challenges, we do not know when we will get additional supply," Alberta chief medical officer of health Deena Hinshaw said on Twitter.
The province has administered roughly 255,000 AstraZeneca jabs.
So far, almost 20 million Canadians or 40 percent of the population have received at least one AstraZeneca, Moderna or Pfizer dose.