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Monday, January 30, 2023
Letter: Science and risks of the AstraZeneca vaccine

Dear editor:

In response to a letter by Samuel Young, “Seniors conflicted on AstraZeneca vaccine,” I can understand where a little science might seem intimidating, so I wrote this note to hopefully help to clarify certain issues related to risks.

In Canada the decision on whether a vaccine can be administered is made by Health Canada. In the case of vaccines,they will rely on committees such as the National Advisory Committee on Immunizations as well as their expert panel of internal reviewers.

Health Canada will insist that the benefits of the vaccine must outweigh its risks before it allows it to be used in Canada. This will be done by carefully reviewing the clinical development program.

In the case of the AstraZeneca vaccine there were two randomized placebo controlled trials. In the European trial, which contained 24,000 participants, about 14,000 got the vaccine and 10,000 were put on placebo. In the United States trial, about 32,000 patients were included in the clinical trial: 21,500 patients received the vaccine and 10,500 received the placebo.

If a side effect such as a blood clot occured more commonly in the treated group than it did in the placebo group then it became an adverse event and must be included in the labelling of the vaccine. Health Canada did not notice any such difference in the clinical development program of this vaccine.

Thrombosis Canada states that the incidence of thrombotic events in the general population is about one in 1,000 persons. In a population at the size of Canada this translates to about 38,000 blood clots per year.

None of these people have necessarily received a vaccine. They may, however, have various risk factors that lead them to develop blood clots. This could include the use of certain medications, cancer, prolonged inactivity and genetic conditions. Your physician is the one who can let you know whether you are at higher risk of a blood clot.

Recent reports in Europe have stated that there could be an increase of blood clots with the AstraZeneca vaccine. This was based on reports of 38 blood clots among 20 million people who received the vaccine. This translates to approximately two blood clots per million patients, which is, in fact, much lower than seen in the general population.

Can we say that the vaccine prevented blood clots? Most certainly not. There were five events of cerebral venous thrombosis, which is a blood clot that occurs in the brain. This translates into an incidence of one per 4 million patients.

In a population not treated with this vaccine the incidence of blood clots in the brain (cerebral venous thrombosis) is believed to be between two and 10 per million persons and is a more common condition in patients with blood disorders, pregnancy, hormone replacement therapy, malignancy, systemic infections, irritable bowel disease, in addition to head trauma such as neurosurgery, brain infections.

On March 18, the European Medicines Agency concluded that the AstraZeneca vaccine did not cause an increase in blood clots. On March 16, the World Health Organization made the same comment. On March 17, Dr. Teresa Tam, who is leading the COVID fight in Canada also stated that the vaccine did not seem to cause an increase in blood clots. It would be difficult to argue that there is collusion at play when various regulatory and health authorities come to the same conclusion based on the science.

Health Canada reached the same conclusion on March 24, but said it would add to the labelling a potential blood clot in the brain as a risk factor to this vaccine. This is done so that physicians can be made aware to monitor for this potentially extremely rare adverse event. These serious blood clots are almost always manifested as severe headaches lasting three days or more and can be treated with anti-coagulants.

The risk of a blood clot occurring in a person infected with COVID-19 is about one and 100 cases. The decision on whether to accept the AstraZeneca vaccine can be put in perspective if we look at the various risks in life:

Risk of brain blood clot with AstraZeneca vaccine: 1 in 4 million.
Risk of body blood clot with AstraZeneca vaccine: 1 in 2 million.
Risk of brain blood clot in the general population in Canada: 10 in 1 million. (Thrombosis Canada).
Death by cancer in Canada: 2,108 in 1 million. (Statscan 2018).
Death by cardiovascular disease in Canada: 1,356 in 1 million. ( Statscan 2018).

Blood clot in the general population in Canada: 1,000 in 1 million. (Thrombosis Canada)
Death by COVID-19 in Canada: 600 in 1 million. (As of March 25, 2021).
Death by accident in Canada: 51 in 1 million. (Statscan 2018).
Blood clot in COVID-19 infected person in Canada: 1 in 100. (As of March 25, 2021).

Given that we are in a race between the vaccine and the virus, especially with the new variants, the advantage of preserving immunity and avoiding hospitalization will be given to those who accept the first vaccine offered to them.

The decision on getting vaccinated becomes less intimidating if you compare these various risks. The risk of mortality from COVID-19 by far exceeds the risk of being vaccinated with the AstraZeneca vaccine.

Reports this week from Europe have shown that the vaccine may cause an increased risk of clots in the brain especially among women who are either pregnant, unknowingly, are taking birth control or have recently given birth. This population is typically at higher risk of blood clots because of the increased hormone blood levels.

There have been a few reports in men under age 55 also. These blood clots seemed to occur in patients who had low platelets in their blood. Again this is something you should discuss with your physician if you have some concerns. We only started to immunize patients over 70 years of age as of March 29 in Ontario. 

For these reasons the Canadian National Advisory Committee on Immunizations has stated that the AstraZeneca vaccine should not be used by anyone under age 55. In an abundance of caution, it has decided to reserve the vaccine for people over 60 until more evidence comes to light.

I hope this helps put things in perspective.

Robin Jinchereau


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