Health Matters: Why do blood clots occur in the first place?
Dr. Haythe: Blood clots are formed when platelets clump together to stop the bleeding. There are many different kinds of blood clots, and they can occur when you have damage to your skin or your blood vessels. Your body releases different kinds of clotting factors, like platelets, that help prevent you from bleeding excessively from those small injuries. But they become dangerous when they clump abnormally and don’t dissolve.
Who is most at risk for blood clots?
There are people who have known clotting and bleeding disorders, for example hemophilia, which is a rare, inherited bleeding disorder in which the blood does not clot properly. Aside from these genetic instances, women who take a birth control pill are known to have an increased risk of blood clots in their legs and lungs. Women who are pregnant are also at increased risk of blood clotting, most likely due to hormonal changes in the body. In general, women’s risk of blood clots is higher than men’s because of pregnancy or birth control. In addition, people who are bed-bound or immobilized after surgery, or who have taken a long trip in a plane, car, or train and do not move their legs frequently, are also at risk for clot formation.
What is thrombosis with thrombocytopenia syndrome (TTS)?
Thrombosis is another word for a blood clot. In people with thrombocytopenia, there is a low platelet count, which can lead to excessive bleeding. You might think, well, if blood platelets are low, aren’t you bleeding rather than clotting? But, in fact, they can be low because the platelets are being consumed to clot in a different part of the body.
Is this what happened with the J&J vaccine?
In the case of Johnson & Johnson, 12 of the 15 women developed a type of blood clot that forms in the brain called a cerebral sinus venous thrombosis, which is a form of stroke. These individuals also had a low blood platelet count, or thrombocytopenia. This relatively rare blood clot in the brain is not usually associated with thrombocytopenia, which is why the two conditions combined is extremely rare.
Do experts know why these blood clots occurred?
The CDC has not released much information about other conditions that these people may or may not have had prior to the vaccine, for instance if they had other kinds of clotting disorders. We still need more data, but one hypothesis is that the vaccine caused an inflammatory response that affected the platelets in a way that made people form abnormal clots.
The closest thing we can compare it to is a rare syndrome that occurs in patients who are exposed to a drug called heparin. Heparin is a blood thinner and is meant to thin the blood and prevent blood clots from forming. There are some people who, when given heparin, can develop a very abnormal condition called heparin-induced thrombocytopenia, where the heparin actually causes an abnormal response in the platelets, causing patients to clot and bleed abnormally. This is helpful information since it impacts how people with TTS are treated, and now it is advised that heparin should not be given to patients in this scenario.