Unusual clots have been found in the brains of a few people after they were injected with the Oxford-AstraZeneca vaccine.
These “cerebral venous sinus thromboses (CVST)” have led some countries, including Germany, France and Canada, to restrict who can receive the injection.
The World Health Organization (WHO) and the European Medicines Agency (EMA) say the benefits of the vaccine outweigh any risks.
Scientists and drug safety regulators around the world are trying to find out if the vaccine is actually causing these strokes, how great the risk could be, and what it could mean for vaccination programs.
Is the vaccine causing clots?
At the moment, we do not know.
The EMA, which has been reviewing the safety data, says that “it is not proven, but it is possible.”
The organization has to find out if the clots reported are they a side effect or a coincidence that would have happened naturally.
This is incredibly difficult when it comes to rare events. If, on the other hand, one in 10,000 people had severe blood clots, the answer would be obvious.
I have spoken with respected scientists: some of them are skeptical, others are increasingly convinced.
Some point to the highly unusual nature of clots as a sign that something might be going on.
They often appear together with low levels of platelets in the blood, which are one of the main components of a clot, and with antibodies related to other bleeding disorders that appear in the blood.
Others say there is insufficient evidence and that the reported cases could be due to covid-19, which in turn is linked to abnormal clotting.
How big could the risk be?
It is still entirely possible that the risk is zero, as vaccines have not been shown to cause brain clots.
The Paul Ehrlich Institute of Germany reported 31 cerebral venous sinus thrombosis and nine deaths among the 2.7 million vaccinated there.
However, the most recent data from the UK report far fewer clots, just five, despite having vaccinated far more people – 11 million.
The European Medicines Agency, which has evaluated data from around the world, estimates that the risk of CVST is about one in 100,000 in people under the age of 60 who have received the AstraZeneca vaccine.
It’s “more than we would expect to see,” said the organization’s head of security monitoring, Dr. Peter Arlett.
However, it is not clear what the incidence rate of these blood clots in the brain really is.
Estimates range from around two cases per million people each year to nearly 16 per million in normal times, and the coronavirus itself may also be causing them.
Why is there a difference between the UK and Germany?
You might expect to see the same number of CVTS in different countries if they were actually caused by the vaccine.
However, the UK and Germany have vastly different figures.
One argument is that the profile from the people who get vaccinated is different.
The UK started vaccinating from the oldest age groups, while Germany was one of the countries that initially refused to use the vaccine in those over 65 due to lack of data in the trials.
Instead, almost 90% of Germans who received AstraZeneca are believed to be under the age of 60.
Side effects in general tend to be more serious in younger people, as they have a stronger immune response.
That is one of the reasons that is considered to explain that the United Kingdom has had fewer cases.
However, CVSTs are inherently more common in younger women, and taking the pill increases the risk.
So natural risk levels, whether people are vaccinated or not, could play a role as well.
Separating all of this is challenging, but the EMA indicated that it had not found specific risk factors such as age, gender or medical history.
Is the AstraZeneca vaccine safe?
Nothing in medicine is completely safe and even highly toxic therapies are used in certain circumstances.
Chemotherapy drugs have brutal side effects, but they are very valuable.
And even over-the-counter pain relievers like acetaminophen and ibuprofen have serious, if incredibly rare, side effects.
The real decision it is always whether the benefits outweigh the risks.
This is a particularly great challenge in a pandemic. Normally, medicine would be based on the “precautionary principle” to demonstrate adequate safety before administering a new medicine to large numbers of people.
But in a pandemic, any delay in vaccinating populations will also cost lives.
Just based on data from Germany, of the 1 million people vaccinated, 12 would be expected to have a blood clot and four of them would die.
But if one million 60-year-olds contracted the coronavirus, around 20,000 would die from covid-19.
Along these lines, if a million people in their 40s were infected with the coronavirus, around 1,000 would die. And in the case of 30-year-olds there would be a few hundred deaths.
The benefits of vaccination clearly increase with age and countries such as Germany and Canada have allowed the AstraZeneca vaccine to be used in older population groups.
These decisions will also depend on what alternative vaccines are available and who remains to be immunized.
The world is examining the data carefully, but it will still take time to have a clear answer.
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