After distributing more than 2.3 million AstraZeneca vaccine doses nationwide, public health officials have so far identified 28 suspected cases of a rare but serious condition called vaccine-induced thrombotic thrombocytopenia (VITT).
Dr. Howard Njoo, Canada's deputy chief public health officer, said Thursday that 18 of those cases have been verified through testing while 10 more await laboratory confirmation.
VITT is blood clots combined with low levels of blood platelets following immunization.
According to the Ontario Science Table, VITT can present itself four to 28 days after vaccination, so reports of more cases are likely by month's end. There have been four VITT-related deaths reported in Canada.
"We take these reports of side effects, however rare, very seriously and we consider these carefully when looking at the vaccine portfolio in Canada," Njoo said.
WATCH: Health officials give update on AstraZeneca risks and the timing of second doses
The report on VITT cases comes after some provinces have suspended the use of the AstraZeneca shot. Alberta, New Brunswick, Nova Scotia, Ontario, Saskatchewan and Quebec have all said they will stop using the vaccine for first doses, at least for now.
In announcing its suspension, Ontario said it now believes the VITT incident rate is roughly 1 in 60,000 shots administered — a rate that is much higher than the figure previously cited by Health Canada.
While the AstraZeneca vaccine has been put on pause in many jurisdictions, Canada continues to procure tens of thousands of doses of this product. COVAX, the global vaccine sharing initiative, delivered 665,000 doses today. Those doses have a three-month shelf life and must be administered by the end of August.
Njoo said Canada will continue to buy enough vaccines to get existing AstraZeneca recipients a second shot.
According to Health Canada, at least 1,540,000 AstraZeneca doses have been administered in Canada as of May 1. Thousands of Canadians have been vaccinated since then.
"The situation with AstraZeneca is evolving and we will work with the provinces in terms of their supply needs," said Maj.-Gen. Dany Fortin, the military commander leading vaccine logistics at the Public Health Agency of Canada.
"We want to assure everyone that sufficient supply will be available for those who want a second dose of the AstraZeneca or who cannot take an mRNA vaccine. Provinces are thinking about ordering doses for second doses to immunize those who've had a first dose. That will start in the next few weeks."
Some provinces, notably Ontario, have put all AstraZeneca vaccinations on hold, including second shots.
"We need to know more before we deal with any more doses of AstraZeneca being put into arms," said Christine Elliott, Ontario's health minister.
"We are waiting for a determination by Health Canada as well as NACI on the situation with respect to VITT and the safety overall," Elliott said, referring to the National Advisory Committee on Immunization.
Dr. Jennifer Russell, New Brunswick's chief medical officer of health, said AstraZeneca will continue to be offered as a second dose and to people who aren't able to get out to a clinic that offers the mRNA vaccines from Pfizer and Moderna.
Early data from the United Kingdom suggest the risk of VITT after second doses of AstraZeneca is likely lower than the risk after first doses.
Njoo said there is still time for provinces and territories to figure out what to do about second doses for AstraZeneca recipients.
The vaccine maker recommends at least a 12-week interval between the first and second shot. Canada didn't start administering AstraZeneca until the second week of March, meaning those patients who got an early dose won't be due until the end of May for the second booster shot.
Other options could be made available to Canadians based on the results of an ongoing U.K. study.
Oxford University is studying a "mismatched" vaccine regimen by testing the results of giving one dose of AstraZeneca followed by a Pfizer booster shot.
While the trial data have not yet been released, some scientists have suggested such a mix could produce a stronger immune response than two doses of the same product. Early data suggest, however, that there may be more reactions to this sort of mix-and-match regimen, such as fatigue, headache or fever.
"Generally, vaccine series are completed with the same vaccine, but in some cases they can be completed with a different one," Njoo said.
"It may be possible that using a different vaccine type could result in a greater immune response, providing broader protection against COVID-19."
Canada reports 28 cases of rare blood clots following AstraZeneca vaccinations - Yahoo Canada Shine On
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