Do all COVID Vaccines have the same Blood Clot Risk?

6 minute read

So, I’m excited today, because I just got my booking for the Astra-Zenica vaccine for this Friday. For context, I’m over 40 and living in Ontario.

So why am I excited? Well, because until today our province had been limiting vaccine access to people 55 or older, and 50 in some cases or 18+ for very high risk areas. Apparently, the Ontario government was under the impression there were not allowed to go against the advice of the National Advisory Committee on Immunization (NACI) and give doses to the general population in lower risk areas until the Federal Government approved it. This weekend, the federal government clarified to them that this was never the case, each province can take the NACI advice as they see fit.

So now Gen X is jamming the twitter machines, waitlist apps and phone boards to get their vaccine, as well as discussing the ethical and moral implications of doing so when you know others are more at risk than you. The general verdict of these discussions experts in medicine and disease spread seems to be, if you can get the vaccine, and you are not directly taking it from someone at higher risk, then take it! Each person who gets vaccinated, no matter what age or risk level, brings us one step closer to Herd Immunity, which reduces everyone’s risk.

In Ontario, this means everyone over 40 should try right now, Shopper’s Drug Mart has a great website registration process. Rexall’s is fine too, although you have to add yourself to a waitlist for each individual pharmacy location. I’m not complaining though, I added myself last night and this morning received a bunch of texts for signing up and got one for Friday at my local Rexall. My wife called another indepenent pharmacy locally and got an appointment for tomorrow! So try different approaches. In BC, they are going even further, ramping down the age one decade a week until all adults are eligible. This is the kind of urgency we need. If enough people sign up we should even consider openning 24 hours vaccine clinics for a while to let people get it quicker, assuming there is enough supply.

So what about Blood Clots?

With this new influx of people will come the question of which vaccine to get. Pharmacies in Ontario are giving out only Astra-Zenica right now, so that’s what I’m getting.

“But doesn’t that cause blood clots?”

Did you say “cause”? I’m so glad, because that’s a very specific word, on which I have opinions. The short answer is, “we don’t know”. The slightly longer answer is, “maybe, but so do a lot of things”. The really long answer, science takes a long time, we’ll tell you in 5 years.

On this topic there’s a recent post on MarketWatch that summarizes a study from a psychology group at Oxford:

Taquet, M. (2021, April 15). COVID-19 and cerebral venous thrombosis: a retrospective cohort study of 513,284 confirmed COVID-19 cases. –

This study looks at existing records from the UK NHS health network since January 2020. They use that data to analyse the number of people who experience the severe blood clotting that everyone is worried it about.

What Kind of Blood Clot?

Blood clots are apparently a standard, but very rare, expected effect from vaccines and flu and coron style viruses. Blood clots can also be associated with age, with pregnancy, birth control pills and many other factors. But people should be aware that the blood clots being talked about related to COVID and the vaccines is a much more severe kind called Cerebral Venous Thrombosis which kills almost half the people who experience it. So it is a very serious condition. That being said, it is extrememly rare. And I mean, so rare, that you’d never buy a $10 raffle ticket from someone with those odds because you know you’d never get it back.

It is also sometimes called called cerebral venous sinus thrombosis (CVST). I’m not an expert on the expedemiology, so I don’t know if these are actually different things, the second reference describes CVST with regards to strokes cause by the blood clots, whereas as others references include a varety of less severe conditions in CVT or CVST as well. I’ve broken out the numbers seperately in order to be cautious.

What did this study look at?

The study looks very clear and straightforward, but keep in mind, it is not a study experimenting on people so see a causal effect. That is, it is not the kind of study we all know about from testing the vaccines themselves for safety and effectiveness. In those studies, thousands of participants are carefully selected from the population, some are given a vaccine, some are not, and they are watched for months to see if they catch the disease randomly in their daily life.

So what did they find?

The headline result of their study is summarized in the first four rows of the table below, that the other two major vaccines, Pfizer and Moderna, seem to have the same risk of CVT as the Astra-Zenica vaccine which is being investigated and delayed in many countries in Europe. On top of that, the rist of blood clots from COVID-19 itself is much higher.

Event Odds of Getting CVT/CVTS Odds of having CVST Caused Stroke Cases
Pfizer Vaccine (BNT162b2) or Moderna Vaccine (mRNA-1273) 4 in 1 million Unknown 2 CVT cases out of 489,871 people vaccinated
Astra-Zenica Vaccine (ChAdOx1) 5 in 1 million   169 CVT cases out of 34 million vaccinated in Europe
Catching COVID-19 39 in 1 million   20 CVT cases out of 513,284 people with COVID-19
Catching the Flu 0 in 1 million   No CVT cases out of 172, 742 people with influenza
Being a Living Adult Over 18   5 in 1 million about 10,500 per year in USA
Being a Living Youth Under 18   10 in 1 million about 3,200 per year in USA

Possible problems with this paper’s analysis

I’m no expert in this field, but from a pure data analysis point of view, there are some issues with this study. The authors don’t report their results for Astra-Zenica in the same way they do the mRNA vaccines, so it’s hard to comapre. The “5 in 1 million” results comes from a Europe-wide report they were not involved with. They say in their own data that .4 people per million over any 2-week period. It’s not clear why this 2-week period caveat isn’t present in the other data.

Another issue that the media might miss is that the number of cases of people with CVT is so small (20 people, or 2 people, that we cannot generalize to the larger population based on it). It’s just too small. So, this might be giving us evidence the risk is the same, or it might be noise.

Whatever the answer, it’s clear the risk of this health event is very low and importantly, much less than from the COVID-19 disease itself. Given the exponential growth of the virus right now in many parts of Canada, your risk of getting this disease and suffering some very harsh short or long term effect is much higher than the tiny chance of getting this rare, but serious, blood clotting condition.

So, please consider that and get vaccinated as soon as you can, for all our sakes.