If you follow COVID vaccine news as closely as I do, then the link between the AstraZeneca vaccine and blood clots is probably something that’s made its way onto your radar screen.
It’s been in the news intermittently for months - so much so that AstraZeneca put out a press release in March trying to reassure people that their vaccine is safe.
But the news reports just haven’t gone away, and in a couple of papers just published in the New England Journal of Medicine, we can learn some useful information about this scary topic.
To start out - the blood clot issue is *not* why the AstraZeneca vaccine hasn’t been approved in the United States. Their shoddy trial execution is responsible for the lack of FDA approval.
But the blood clot issue is concerning. And since this vaccine has already been given to millions of people and likely will be given to millions more, I thought it was worth learning about for the newsletter today.
There are two reports - one from Germany and Austria as well as one from Norway - that can help illuminate things here. Let’s take a look.
The story begins after a healthy 49 year old woman died from complications associated with vaccination
The AstraZeneca Covid vaccine (abbreviated ChAdOx1, which comes from the fact that it is a chimpanzee adenovirus vector) has been given to approximately a quarter of the vaccine recipients in Germany and Austria.
While this report details 11 patients from Germany and Austria who had events related to blood clotting, the authors start with the index case that spurred investigation.
A 49 year old health care worker with no medical problems received the vaccine. She felt minor symptoms for a few days, then started to develop more severe symptoms by day 5, and was admitted to a local hospital by day 10.
She developed blood clots in several different places: the veins leading to her liver (portal vein thrombosis), her lungs (pulmonary emboli), and the veins inside her skull (cerebral venous thrombosis). She ultimately died after having a bleeding complication related to started blood thinners for treatment of these blood clots.
This was in February.
By March 15, another 10 cases of blood clotting related complications had been found. There were 9 blood clots in the brain. Of note, a cerebral venous thrombosis is a different type of blood clot in the brain than a standard stroke. This is a blood clot forming in the veins that drain blood out of the brain rather than a blood clot in the arteries bringing blood to the brain (what usually causes a stroke).
Of these 11 patients, the median age was 36 and the majority were women.
These adverse vaccine effects occur in young, healthy people - and we’re talking about really bad outcomes.
The blood clotting that we’re seeing here isn’t trivial.
In the Austria-Germany cohort, 6 of the 11 women who suffered blood clotting events subsequently died from them. These are almost certainly deaths caused by vaccine-related complications.
In the Norweigian cohort, outcomes were similarly alarming. The Norway group was also comprised of young, healthy people.
They looked at 5 cases of vaccine-related blood clots, 4 of whom had clots happen in their brains. These clots were cerebral venous thromboses - just like the Austria/Germany group.
3 patients in this cohort died.
So these two reports comprise 16 patients, 9 of whom died from blood clotting related complications.
The biology of these clots is something that we’ve seen before in medicine
When patients are admitted to the hospital, we often give a medicine called heparin to prevent blood clots. Heparin is a blood thinner that’s administered to millions of patients everyday.
Unfortunately with heparin, patients will rarely develop a blood clotting complication called “heparin induced thrombocytopenia” or HIT.
HIT develops due to an autoimmune reaction that activates platelets, a type of blood cell involved in blood clotting. A similar reaction has been seen in patients who never received heparin, which is termed autoimmune heparin induced thrombocyotpenia (a bit of a misnomer since no heparin is involved).
Blood samples taken from patients who develop clotting disorders show activation of something called “platelet factor 4” (PF-4), which is activated in HIT.
It appears that the vaccine causes an autoimmune reaction that activates PF-4 and causes blood clots.
As a consequence, researchers are calling this problem “vaccine-induced immune thrombotic thrombocytopenia” or VITT.
Before you start drawing conclusions about stopping this vaccine, keep the total numbers and context in mind
The AstraZeneca vaccine has been given to about 34 million people and there have been 222 blood clots reported (169 in the brain and 53 in the veins in the abdomen).
That means these blood clotting events happen 0.00000653% of the time.
Now, let’s do a bit of back of the envelope math to think about how this compares to a bad outcome from COVID. If you think of the case fatality rate as about 1% overall, and assume that only 10% of infections become cases and that the fatality rate is an order of magnitude lower for young people, you still get a 0.01% chance of dying from COVID, or about 1 in 10,000.
That means that your chance of a potentially fatal blood clot from vaccination is about 1 in 150,000 while your chance of dying from COVID as a young, healthy person is about 1 in 10,000.
Ultimately, it’s important to keep in mind that COVID is still bad. But this changes things
The patients who develop VITT suffer terrible consequences. You can make a real argument that no one should get this vaccine because of the concern of creating blood clots in healthy people.
But COVID remains bad and avoiding it remains good.
That said, I wouldn’t get this vaccine now. And I wouldn’t advise my patients, family, or friends to get it either.
The individual risk-benefit analysis is certainly different with this vaccine than with Pfizer and Moderna.
The Johnson & Johnson vaccine (also an adenovirus vector like AstraZeneca) now has some similar concerns with 6 cases of blood clots in the US reported after 7 million US doses have been given.
That’s why we are pausing J&J distribution here in America.
No reports have been published that I’ve been able to find to see whether these are similar to VITT, but when I’m able to learn more I will certainly cover it in this newsletter.
The good news here is that this safety signal isn’t present with the Pfizer and Moderna vaccines. So we still have two excellent vaccine options available, with production increasing by the day.
More to come on J&J when it’s available.
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