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A man looks up at an electronic COVID-19 vaccination sign at Dundas Square during the COVID-19 pandemic in Toronto on May 11.

Nathan Denette/The Canadian Press

Ontario will stop the use of the Oxford-AstraZeneca COVID-19 vaccine for first doses after the province tracked a recent increase in the incidence of a rare but serious blood clot condition linked to the shots, the second province after Alberta to put the vaccine on hold. Saskatchewan also said on Tuesday the province is no longer using AstraZeneca vaccines for first doses due to lack of supply.

The federal government said it remains committed to bringing in more AstraZeneca doses as it tries to speed up the pace of vaccination. On Tuesday, Canada hit a new milestone when it topped the Group of 20 in the average daily rate of vaccines administered per capita.

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Officials in Ontario said recent data showed the incidence of the blood-clot syndrome shifted from one for every 100,000 people vaccinated with AstraZeneca to one for every 59,000 people vaccinated with the shot. Health officials said they decided to act as the risk of COVID-19 has also started to drop in Ontario – and supplies of other vaccines now far outnumber those of AstraZeneca.

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Ontario Chief Medical Officer of Health David Williams said the province was acting out of “an abundance of caution” because of an increase in severe blood clots linked to the vaccine reported in just the past few days. Officials said the increase has also been observed in other countries, as more of the vaccine had rolled out and more side effects have been reported.

They were not able to say whether the 853,885 Ontarians who already got their first shot of AstraZeneca will receive a second dose of the same vaccine or something different. Shots from Pfizer-BioNTech and Moderna are mRNA vaccines that are not connected to the blood-clot syndrome.

The province has administered 651,012 doses of AstraZeneca, and 202,873 doses of the made-in-India version of the shot known as Covishield. The new increase in blood clots, Dr. Williams said, is largely related to the brand-name AstraZeneca doses.

Health officials cautioned the side effect remains rare. In all of Ontario, there have been eight cases, said Jessica Hopkins, the chief health protection and emergency preparedness officer with Public Health Ontario. But she called the change in the rate of side effects “a significant safety signal that we don’t want to ignore.”

On Monday, Alberta was the first province to stop using AstraZeneca for first doses.

Prior to the announcement from Ontario, Prime Minister Justin Trudeau on Tuesday continued to urge people to take the first shot they are offered and said all vaccines distributed in Canada are safe.

The public-health advice in Canada is that second shots match first shots, but Dr. Williams said Ontario is reviewing the future use of AstraZeneca. He said data from Britain suggest the risk of a blood clot after a second dose was much lower. Officials are awaiting information from a British trial on mixing vaccines, before making any changes to advice on second doses in Canada. Before Ontario announced the pause with AstraZeneca, Chief Public Health Officer Theresa Tam told reporters in Ottawa that updated advice would be available before individuals are due for their second shot.

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Dr. Williams said that those who got their first shot of AstraZeneca should not feel like they made a mistake.

“Based on the much higher risk of COVID-19 infection recently observed in Ontario, including hospitalization and serious illness and death, we maintain that those who received their first dose of the AstraZeneca vaccine did absolutely the right thing to prevent illness and to protect their families, loved ones and communities,” Dr. Williams said.

According to Britain’s drug regulator, the incidence of the syndrome, called vaccine-induced immune thrombotic thrombocytopenia (VITT), is about one in 100,000. But other studies in Europe put the incidence at one in 26,000. The Public Health Agency of Canada did not provide an updated national rate on Tuesday. As of April 28, the rate of VITT in Canada was one case for every 100,000 people vaccinated with AstraZeneca.

On Monday the agency said 12 cases of VITT have been reported in Canada, and three people have died.

There is a lag between the first shot and when individuals develop the rare blood clot. Because of that, the new incidence rate reported by Ontario was not calculated based on the total number of AstraZeneca shots administered by Tuesday, but rather on the amount of vaccine administered on April 24, the day after the most recent date connected to a reported blood clot.

Cases of the syndrome have been reported between four and 28 days after vaccination. Public-health officials say individuals should seek immediate medical help if they develop symptoms including shortness of breath, chest pain, persistent abdominal pain, and severe or worsening headaches and blurred vision.

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Dirk Huyer, Ontario’s chief coroner and the province’s co-ordinator of outbreak response, said the change would not affect the province’s goal of getting first doses to 65 per cent of Ontarians by the end of the month, as this benchmark relies largely on its increasing supply of the Pfizer vaccine. He said Ontario did not have a confirmation on how much more AstraZeneca it would receive in the future or when. It has about 50,000 doses left.

The federal government has already confirmed that 655,000 doses of the shot will arrive in the coming weeks and another one million AstraZeneca doses will arrive in June.

The daily vaccination pace, now the fastest among the world’s richest countries, is driven mostly by the Pfizer and Moderna vaccines. The federal government is counting on the AstraZeneca shot to ensure the maximum speed of Canada’s vaccination campaign, but it has bought enough mRNA vaccines to still complete vaccinations without AstraZeneca.

“From the federal government’s perspective, our job is to make sure that we get the largest number of safe vaccines into Canada as quickly as possible,” Mr. Trudeau said.

Intergovernmental Affairs Minister Dominic LeBlanc said “it would be irresponsible” for Canada not to pursue the next deliveries of the AstraZeneca shots, as it tries to speed up the vaccination campaign across Canada.

Canada hit the milestone in its vaccination rate as the Prime Minister urged premiers to keep public-health restrictions in place until the majority of Canadians have their first shot.

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“We can’t ease public-health restrictions until cases are way down,” Mr. Trudeau said Tuesday. “As Dr. Tam said, at least 75 per cent of Canadians need to have received their first shots.”

In late April, the Public Health Agency of Canada released modelling that showed provinces and territories need to reach 75-per-cent first-dose coverage among adults and 20-per-cent second-dose coverage to safely lift restrictions without risking a devastating fourth wave.

With reports from Les Perreaux and Ivan Semeniuk; files from Canadian Press

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