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What do we know about the Oxford-AstraZeneca COVID-19 vaccine?

The messaging around the Oxford-AstraZeneca COVID-19 vaccine hit another roadblock Monday as the National Advisory Committee on Immunization said those inoculations should be paused for people under 55 due to concerns it may be linked to rare blood c

The messaging around the Oxford-AstraZeneca COVID-19 vaccine hit another roadblock Monday as the National Advisory Committee on Immunization said those inoculations should be paused for people under 55 due to concerns it may be linked to rare blood clots.

Health Canada is also demanding AstraZeneca do a detailed study on the risks and benefits of its COVID-19 vaccine across multiple age groups after getting more reports that patients in Europe developed blood clots following vaccination.

NACI says people 55 and older can still be given the vaccine with consent, after informing them of potential risks. There have been no reports of blood clots in Canada.

The worldwide cases identified so far have primarily been in women under the age of 55, Dr. Shelley Deeks of NACI said in a press conference on Monday, and they have mostly occurred between four and 16 days after receiving the vaccine.

This isn't the first time recommendations around AstraZeneca and Covishield, the brand name for batches produced by the Serum Institute of India, have changed.

Days after the vaccine was first approved by Health Canada for all adults on Feb. 26, NACI said it did not recommend AstraZeneca for people over 65, citing not enough evidence from clinical trials to show whether the vaccine was effective in older populations. But NACI reversed its decision on March 16, saying real-world data suggests the shot was safe and effective among seniors.

Around the same time earlier this month, some European countries temporarily halted use of the vaccine due to reports of blood clots. The vaccination campaigns have since resumed in most of those countries after a review by the European Union's medical regulator.

Health Canada last week updated the product label on the vaccine to warn about blood clotting, saying reports of those events are "very rare."

Here's what we know about the AstraZeneca vaccine:


The rare events being seen in some AstraZeneca recipients is called Vaccine-Induced Prothrombotic Immune Thrombocytopenia (VIPIT), which can cause death but can also be treated if caught early, NACI said.

Deeks said Monday that original estimates for VIPIT among AstraZeneca recipients were one per one million people, but a higher rate of one per 100,000 has been reported by Germany.

She added that 40 per cent of those with the syndrome have died.

Health Canada's chief medical adviser Dr. Supriya Sharma said last week she agrees with European health authorities that the benefits of the vaccine outweigh any potential risks, and that all four vaccines approved for use in Canada were considered safe.

She added Monday, however, that may not be the case in all age groups, but there isn't enough data to say for sure.

Since younger individuals are less likely to suffer severe COVID outcomes, Sharma says it may make sense to wait for more data on AstraZeneca before giving them the vaccine.

"I think the recommendations ... are really reflecting the risk of having COVID in those younger age groups," Sharma said. "So we're looking at a rare risk of this potential adverse event — this severe clotting event with low platelet counts — compared to what your likelihood of catching COVID and actually having a (severe) outcome."

Germany's medical regulator said Monday it has received reports of 21 cases of rare blood clots in people who had recently received AstraZeneca's coronavirus vaccine. The Paul Ehrlich Institute told The Associated Press that seven people affected by the blood clots have died.

Of the 21 cases, 19 were in women ages 20 to 63, while two were in men ages 36 and 57.

During the period covered by the reports, some 2.27 million first doses of the AstraZeneca vaccine were administered in Germany.

Dr. Howard Njoo, Canada's deputy chief public health officer, said that while rare, the clotting issue needs "further evaluation."

"Certainly we're looking to the experts at NACI and others to evaluate ... the evidence and then give further guidance."


Data from clinical trials showed AstraZeneca was 62 per cent effective in preventing COVID infections, but it also prevented death and hospitalization in all participants who got the virus after receiving the vaccine.

Efficacy was a major talking point when AstraZeneca was first approved, with some comparing it to the 95 per cent efficacy shown in mRNA vaccine trials from Pfizer and Moderna. But experts have stressed that all the authorized vaccines offer excellent protection against severe disease.

Real-world data is emerging that may also suggest the efficacy of AstraZeneca's vaccine increases over a longer time interval between the first and second shot. Clinical trials used a four-week span between doses but some countries have been delaying second doses by several weeks. In Canada, many provinces have opted to delay the second dose by four months.

AstraZeneca's vaccine also had some promising data last month suggesting it works against the variant first detected in the U.K. Findings based on swabs taken from around 500 volunteers in trials between October and January showed a 74.6 per cent efficacy rate against that variant.

AstraZeneca can be shipped and stored at regular refrigerator temperature, unlike Pfizer-BioNTech and Moderna, which need colder storage temperatures.

From a global vaccination standpoint, the low cost of AstraZeneca's vaccine — about US$4 per dose — gives it another advantage. AstraZeneca, which says it aims to manufacture up to three billion doses in 2021, has pledged to make their product available at cost around the world until at least July.


Unlike Pfizer-BioNTech and Moderna, which use messenger RNA (mRNA), the AstraZeneca vaccine is a non-replicating viral vector, using a weakened chimpanzee cold virus as a vessel.

Scientists stripped the genes from that virus, which isn't harmful to humans, and replaced them with the spike protein gene for SARS-CoV-2.

Once injected, the vaccine shows our bodies how to produce the immune response needed to ward off future infections from the COVID-19 virus.

Some may see outward signs of an immediate immune response to the vaccine — the body's way of preparing for what it perceives as an attack by the virus. This can cause side effects usually seen with other vaccines, including pain at the injection site, redness, swelling and even fever, but experts say that means the vaccine is working.

—With files from The Associated Press

This report by The Canadian Press was first published March 29, 2021.

Melissa Couto Zuber, The Canadian Press

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