After safety review, Health Canada says people shouldn't hesitate to get AstraZeneca vaccine

Following reports of rare blood clotting events among some who have received the AstraZeneca vaccine, Health Canada officials said today they still believe the product is safe — and Canadians should have no qualms about rolling up their sleeves for it when the time comes.

Speaking to reporters at a technical briefing, Dr. Supriya Sharma, Health Canada's chief medical adviser, said there is a "very low risk" that some patients could develop blood clotting with low platelets following vaccination.

Sharma said that with a third wave of COVID-19 raging — and with the risk of hospitalization and death still much greater than the risk of developing clots following an AstraZeneca shot — the benefits of this vaccine clearly outweigh any risks.

"Get whatever vaccine is available to you. It's that simple. The longer you wait to get vaccinated, the longer you're not protected," Sharma said. "We know the risks of getting these side effects from the vaccine are very rare."

WATCH: Health Canada's Dr. Supriya Sharma on the AstraZeneca vaccine

Pointing to evidence from the U.K., where the AstraZeneca shot has been administered 20 million times, Sharma said the chance of developing these clots is roughly 1 in 250,000.

While there are other numbers circulating about the frequency of these clots, Canada is relying on the British data because the U.K. has a robust safety monitoring system and has administered a lot of shots, Sharma said.

Meanwhile, Sharma said, the odds of developing a "regular" clot — and not this rare, vaccine-induced thrombocytopenia (VIPIT) in people with low platelets — are 1 in 5 for people hospitalized with COVID-19.

The risk of a woman between the ages of 15 and 45 developing any sort of blood clot is 1 in 3,300. If they take birth control, the odds are 1 in 1,600. If a woman is pregnant, the risk is 1 in 300, Sharma said.

"It is difficult to wrap your head around these abstract numbers and relative risks. We understand that," Sharma said. "The risk of regular clots with COVID is much, much higher and it's much, much higher than having this very severe clot."

Health Canada still has not definitively associated the AstraZeneca vaccine with these clots. Sharma said only that the shot was "probably" the reason why some people developed this condition.

WATCH: Canadian lab studies clotting risk

Sharma said the department has updated the product label to warn would-be patients about the risk of developing these clots. Patients who receive the shot will be told to look out for symptoms — severe headaches, abdominal pain, leg pain or shortness of breath.

Sharma said that if regulators conclude at any time that the risks of the AstraZeneca shot outweigh its benefits, Health Canada won't hesitate to pull the vaccine's authorization.

She said it's not unusual for such very rare side effects to emerge when a vaccine has been given to such large numbers of people.

One woman in Quebec has developed clots after receiving the Covishield product, a biologically identical version of the AstraZeneca vaccine produced by the Serum Institute of India. The woman — who is over the age of 55, according to Health Canada — is recovering at home.

Sharma said these rare blood clots can be diagnosed and treated.

Other authorized products widely used in the medical profession can present similar rare clotting risks.

Heparin, an anticoagulant (blood thinner) medication sometimes used in the treatment of heart attacks and angina, can lead to thrombocytopenia (low platelet count), Sharma said. The possibility of rare side effects doesn't render a product useless, she said.

"Heparin medication, that seems to trigger the same type of reaction. We still have heparin on the market, we still use it for prevention," she said.

In approving the vaccine in late February, Health Canada regulators authorized its use for all adults 18 and over.

WATCH: Health Canada says people shouldn't hesitate to get AstraZeneca vaccine

Sharma said Health Canada still believes that the shot can be safely deployed in all adults regardless of age. However, the National Advisory Committee on Immunization (NACI) has recommended that provinces restrict access to adults over the age of 55 — for now.

Sharma said NACI is reviewing more real-world data now and could update its guidelines.

"NACI is looking at it. We've presented our data to them. They will provide a recommendation and that may include a change in age — they're re-looking at the age," Sharma said.

A recent study from the Cambridge University-based Winton Centre for Risk and Evidence Communication found, after crunching the number of vaccine-related injuries, that the benefits of a AstraZeneca shot easily outweigh the potential risks in virtually every age cohort and among people facing low, medium and high risks of exposure to COVID-19.

The shot has kept thousands of people out of hospital and has pushed down ICU admission rates dramatically in the U.K., the study found.

Statistician David Spiegelhalter, director of the Winton Centre, said the risk of developing a blood clot is "tolerable."

"For most people, when there is virus circulating, the risks of COVID-19 outweigh the minimal risks from the vaccines," Spiegelhalter wrote in his report.

"What else has roughly a one in 100,000 chance for a young adult? We could choose from the risk of dying when under general anaesthesia, or in a skydiving jump, or, on the positive side, winning the Lotto jackpot if you bought 450 tickets, or guessing the last five digits of someone's mobile phone number," Spiegelhalter wrote.