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Pharmacist Barbara Violo arranges all the empty vials of the Oxford-AstraZeneca COVID-19 vaccines at the Junction Chemist during the COVID-19 pandemic in Toronto, on April 19, 2021.Nathan Denette/The Canadian Press

Ontario will start using its soon-to-expire supply of the AstraZeneca vaccine for second doses next week after putting the shots on hold over concerns about rare severe blood clots, the province’s Chief Medical Officer of Health says.

David Williams announced on Friday that a review of data on the number of severe blood-clot side effects expected from second doses of AstraZeneca in Britain shows a rate of one in 600,000 people – higher than previous estimates of one in a million, but still “very, very rare.”

Several provinces have halted the use of the shot for first doses. Ontario was facing a deadline for its decision on second doses: It has about 55,000 shots on hand, some of which expire May 31. Ontario Health Minister Christine Elliott had initially suggested that despite best efforts, a small number of those doses might end up discarded. But she has since said none would go to waste. Another 250,000 doses of AstraZeneca were to arrive in Ontario this week.

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Dr. Williams said Ontarians who received their first doses between March 10 and March 19 can now book a second dose for next week. The shot was first offered to those 60 and older in pharmacies in Toronto, the Kingston area, Windsor-Essex and in doctor’s offices in some other areas.

This initial group, the government says, could receive their second dose after just 10 weeks, less than the recommended 12-week gap for this vaccine. But the province says this still provides strong protection against COVID-19.

Dr. Williams said people in this group should contact the pharmacy or doctor’s office where they received their first dose to schedule a second one. He said others seeking a second shot of AstraZeneca, who received their first dose after March 19, will be offered one within a 12-week window – shorter than the current 16-week interval for other vaccines – soon.

Ontario and the National Advisory Committee on Immunization are still waiting for more data on whether a second dose of a different vaccine offers enough protection.

Some medical experts warn that the British estimate of the risk of a severe side effect from a second dose of AstraZeneca, based on 15 cases out of nine million second doses administered, was bound to rise as more cases surface in the coming weeks. The rare blood clots, which can be fatal or leave patients severely disabled, can emerge up to 28 days after a shot.

When Ontario suspended the use of the vaccine for first doses last week, it estimated the risk of a severe blood clot from an initial shot at about one in 60,000, which was higher than initial estimates of one in 100,000. But data compiled last week by Ontario’s COVID-19 Science Advisory Table shows the risk of severe blood clots from first doses in other countries has varied: In Norway, which had five total cases, it was one in 26,500. In Australia, which had 11, it was one in 127,300.

Ontario, which has administered nearly a million doses of AstraZeneca, has recorded 11 cases of what doctors call vaccine-induced immune thrombotic thrombocytopenia (VITT), and 14 cases of similar severe blood clots in people who had the shot but whose condition has not been confirmed with testing, according to Public Health Ontario.

As of last week, there were 28 cases either confirmed or under investigation in Canada, and three deaths, with about two million shots administered.

Andrew Morris, an infectious-disease physician at Toronto’s University Health Network and a member of Ontario’s science table, said Ontario should instead have sent its leftover AstraZeneca to other countries where the risk of dying from COVID-19 is still high and the safer Pfizer-BioNTech or Moderna vaccines, which are plentiful here, are unavailable.

“My wish is that it should be taken off the market in Canada,” Dr. Morris said. “That’s my personal belief. They should have done this weeks ago.”

Justin Bates, CEO of the Ontario Pharmacists Association, said the announcement was welcome, but still represents a “logistical challenge” for pharmacies, which will have to move quickly. He said pharmacists are reaching out to those eligible for the second shot but that many have already been inundated with requests for the vaccine.

The remaining doses will have to be transported to the 325 pharmacies that were part of the original AstraZeneca pilot, and he said he expects the doses will start to be administered by Wednesday. Mr. Bates said 31,000 doses will go to the pharmacies, and about 90,000 people received shots between March 10 and 19.

“People can make an informed decision. And I think there are a cohort of people that are very comfortable with getting AstraZeneca,” Mr. Bates said.

New Brunswick, Alberta and British Columbia are all providing second doses of the AstraZeneca vaccine. Quebec used up all of its previous AstraZeneca deliveries on first shots, and is now planning to use its next shipment for second doses for seniors – while also offering a choice of Pfizer or Moderna as a second dose.

Meanwhile, the Ontario government also announced on Friday that long-term care homes would start allowing outdoor visits with residents as of Saturday. The government said homes would allow two visitors, in addition to two essential caregivers, to visit residents outside.

In addition, the province said it was extending its provincewide vaccine booking system to those 12 to 17 years old more than a week ahead of schedule, and would allow them to make appointments to receive the Pfizer vaccine starting at 8 a.m. on Sunday.

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