Ontario pauses administration of AstraZeneca vaccine as a first dose

Ontario is pausing the administration of the AstraZeneca vaccine as a first COVID-19 vaccine dose, Ontario's chief medical officer of health announced Tuesday, citing an "abundance of caution" concerning an observed increase in cases of rare, but serious blood clotting in the last few days.

"We maintain that those who received their first dose with the AstraZeneca vaccine did absolutely the right thing to prevent illness and to protect their families, loved ones and communities," said Dr. David Williams at a late-afternoon media briefing.

Ontario initiated the pause after a similar announcement by Alberta.

The rates of the rare blood clot adverse events, known as vaccine-induced thrombotic thrombocytopenia (VITT), are one in 55,000 and one 60,000 in Ontario, said Dr. Jessica Hopkins of Public Health Ontario. As of May 8, there have been eight cases of VITT in Ontario.

That has changed from earlier estimates of one in 100,000, said Hopkins.

The decision was also based on the increased supply of mRNA vaccines as well as lower case counts, which changes the risk calculation. Dr. Dirk Huyer, on the province's vaccine task force, said the province only has about 50,000 doses of the AstraZeneca vaccine on hand and no additional confirmed deliveries, although the federal government has recently said additional shipments should be expected.

Both Huyer and Williams said the pause will not impact the province's goal of vaccinating 65 per cent of adult Ontarians with their first dose by the end of the month, as that had been based on confirmed shipments of the two mRNA vaccines from Pfizer and Moderna.

The pause does not apply to second doses but the province will provide more guidance on the matter at a later point. Williams said there are early promising results about the mixing of vaccines but also noted that VITT is even rarer after the second dose is given, at about one in a million.

The health officials had little practical advice for people who had recently received the AstraZeneca vaccine and are now concerned, aside from saying health-care providers have been educated about when to look for when it comes to VITT.

It can occur between four and 28 days after the vaccination and can be serious or even fatal, the officials said, but also stressed that cases of VITT remain extremely rare.