COVID-19 vaccine blood clots – the FDA and EMA are doing the right thing

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If you have been watching the COVID-19 vaccine roll-out, you know that blood clots have become a concern with the vaccines from JNJ (Johnson & Johnson-Janssen) and AstraZeneca. Shockingly, some pro-vaccine types are trying to dismiss these concerns by posting ridiculous memes that try to compare blood clots from these vaccines to ones from birth control pills or smoking.

Setting aside the fact that memes are for the intellectually and scientifically weak, such as anti-vaxxers, these memes are trying to compare apples to bowling balls. Furthermore, ignoring these potential links play right into the hands of the anti-vaccine crowd.

mbedIn my not-so-humble opinion, the US FDA, European Medicines Agency (EMA), and other health agencies across the world have done the right thing by “pausing,” or limiting the use of the vaccines from JNJ and AstraZeneca. Science has been doing the right thing with regards to the COVID-19 vaccine blood clots issue.

What is the COVID-19 vaccine blood clots issue?

Let’s start right at the top, before I get into the science of this, this COVID-19 vaccine blood clots issue only relates to the AstraZeneca and JNJ vaccines. The mRNA vaccines from Moderna and Pfizer, which use a different technology than the adenovirus-vector used by the JNJ and AstraZeneca vaccines, are not linked to these blood clots.

Furthermore, the differences between the AstraZeneca and JNJ vaccines are minor – the AstraZeneca vaccine uses a chimpanzee adenovirus, whereas the JNJ uses a human adenovirus.

As I discussed with the AstraZeneca vaccine, the European Medicines Agency (EMA), the FDA for the European Union, believes that there is a plausible link between the AstraZeneca vaccine and a rare but sometimes deadly blood clots. The risk is tiny, but it appears that there is a causal link.

Canada and various provincial health agencies, such as Quebec’s, have issue guidelines regarding the COVID-19 vaccine blood clots issue, especially since the US Government has decided to provide the AstraZeneca vaccine to Canada and Mexico.

The government of Canada stated that:

This adverse event is being referred to as Vaccine-Induced Prothrombotic Immune Thrombocytopenia (VIPIT). This entity is associated with the development of antibodies that “activate” platelets, which stimulate the formation of clots and result in thrombocytopenia. The mechanism of action is similar to heparin-induced thrombocytopenia (HIT). The exact mechanism by which the AstraZeneca vaccine triggers VIPIT is still under investigation. At this time, no other risk factors have consistently been identified in patients who develop VIPIT. This adverse event has not been identified following receipt of mRNA COVID-19 vaccines to date.

And after new data showed a slight increase in the risk of cerebral venous sinus thrombosis (CVST), a very rare thromboembolic event, health authorities in various German states made the decision to temporarily halt vaccinations for younger people after receiving more data about these clots.

Like I mentioned above, the EMA believes that a rare condition called cerebral venous sinus thrombosis (CVST), a clot that stops blood from draining from the brain. Regulators have said it is occurring among those who have received the AstraZeneca COVID-19 vaccine at a rate above what they’d expect to see in the normal population.

The EMA is reporting a total of 169 cases of CVST among the 34 million people given the AstraZeneca vaccine across Europe as of 4 April 2021. Additionally, there have been 52 other cases of rare thromboses. The EMA reported that they based its scientific review on an initial 62 cases and 18 deaths up until March 22, but continued reports did not change their assessment.

In the USA, six cases of CVST have occurred among women below the age of 50 and appeared between one and two weeks after vaccination. This is out of around 7 million doses of the JNJ vaccine.

The key thing to note is that a patient that presents with CVST must be treated differently than individuals with other types of thrombosis. The key characteristic of these clots is a drop in blood platelet counts, which is not observed with clots that form from the use of birth control pills and other causal factors. Thrombolytics (clot-busting drugs) and blood thinners are not indicated for these patients as they may cause a hemorrhage into the brain.

We need to put risk into some perspective, so let’s do the basic math (and please don’t conflate this with real statistics).

The risk of CVST is around 1 per 1 million people who received the JNJ COVID-19 vaccine, slightly higher with the AstraZeneca vaccine. On the other hand, the risk of death from COVID-19 is around 1,000-2,000 per 1 million who contract the disease, which is around 200X greater risk than the risk of thrombosis from the vaccine. (See Note 1)

But there’s more. We do not have information on confounding risk factors – each of these six women may have another risk factor from blood clots to smoking to some other issue.

The risk of CVST in the general unvaccinated population in the USA is around 0.5 to 1 in 1 million, so the observed risk for these types of blood clots after receiving the COVID-19 vaccine is slightly higher than expected.

Yes, the SARS-CoV-2 virus seems to cause thrombi in many patients, which might lead one to believe that there is some biological plausibility to the blood clot issue. We don’t appear to observe these thrombosis events with the Pfizer, Moderna, and JNJ vaccines, so it is possible that either there is an ingredient in the AstraZeneca or JNJ vaccines that leads to this increased risk.

At this time, it’s almost impossible to tell if there is actually a causal link between the JNJ and AstraZeneca COVID-19 vaccines and CVST. We would probably need to see over a billion people vaccinated to come to some statistically solid conclusion.

COVID-19 vaccine blood clots
Photo by National Cancer Institute on Unsplash

The system works

I want to make a few points about why I think the FDA and EMA are doing the right thing regarding the COVID-19 vaccine blood clots:

  1. The anti-vaccine mob thinks that the FDA, CDC, and other regulatory agencies don’t watch for these issues. Yes, there is a tiny risk for CVST, but regulatory agencies saw it and reacted in measure tones (the FDA “pausing” vaccination). This is what they do for all vaccines.
  2. As Orac pointed out, these agencies are caught between a rock and a hard place – if they did something (like they did), the anti-vaxxers will over-exaggerate the issue (which they have) and if the agencies did nothing, the anti-vaxxers will blame them for “hiding” something.
  3. I, and many others, remain unconvinced that there is an actual link between CVST and the two COVID-19 vaccines, but we need to check. There is a curious issue with thrombocytopenia (low platelets) and these clots, which is almost counterintuitive and is not well understood.
  4. The “pause” also gives time to make recommendations to physicians on how to treat this type of CVST – that (1), it should not be treated with blood thinners like heparin, and (2) the association between the CVST and thrombocytopenia may indicate some sort of immune response.
  5. This is how science works, take an observation and create a hypothesis from it – do the COVID-19 vaccines cause CVST. Then analyze the data in a scientific manner to determine if we can accept or reject the hypothesis. That’s what the CDC is trying to do.
  6. All drugs, including vaccines, have risks – what the CDC, FDA, EMA, and others are saying is that the benefits, saving lives, far exceed the risks of CVST. However, if these agencies can make recommendations to reduce the risk, say a certain demographic group with certain other risk factors (say smoking or use of birth control pills) is excluded from using the vaccine, that is a good thing. That’s how the system is supposed to work.

I completely disagree with my peers who are dismissing this risk out of hand because it’s a tiny risk. That plays into the hands of the ignorant anti-vaxxers who wouldn’t know math and statistics if it kissed them on the mouth.

If there is a known risk of CVST, then everyone should know about it. And, although it makes my job hard, I will do the best I can to explain that it’s still safer than getting COVID-19, and it’s still very safe irrespective of the risk-benefit ratio.

The regulatory agencies made the right choices, and the system works.


  1. My calculated numbers for these risks are based on the infamous back of the napkin analyses. Once peer-reviewed papers are published, we will know the precise risk difference between vaccinated unvaccinated groups.


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Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!