Blood Clots: How Those That Follow COVID-19 Vaccination Differ

In this Science Made Easy video, Dr. Paul Offit highlights two major differences between typical blood clots and those that occasionally follow receipt of COVID-19 adenovirus-based vaccines, such as those made by J&J/Janssen and AstraZeneca.


Blood clots: How those that follow COVID-19 vaccination differ

Paul Offit, MD: Hi, my name is Paul Offit. I'm talking to you today from the Vaccine Education Center at the Children's Hospital of Philadelphia. There are three vaccines that are currently available in the United States for COVID-19, for preventing COVID-19. Two of them are mRNA vaccines, meaning messenger RNA vaccines, and one of them, made by Johnson and Johnson, is not. It’s a so-called replication-defective adenovirus vector vaccine, a viral vectored vaccine.

Now, these vaccines are safe. But there's one thing that has come up that is a very, very rare, serious side effect caused by the Johnson & Johnson vaccine, and that is specifically blood clots. It seems to occur … the serious blood clots seem to occur in roughly 1 person per 500,000 that get the vaccine. The blood clots are serious because they can involve the spleen, they can involve areas of the intestine, and they can involve the brain. The brain one is called cerebral venous sinus thrombosis. And it occurs in about 1 per 500,000 people who get the vaccine.

Well, are these blood clots similar to the other blood clots that people can typically get, because the incidence of blood clotting is actually fairly common. Blood clots in the United States occur in roughly 1 per 1,000 people. Usually they occur in the arms or legs, specifically the legs, so-called deep venous thrombosis. Sometimes those blood clots will break off and lodge in the lungs, called pulmonary embolism, and that all can be quite serious. Usually the risk factors are things like cigarette smoking, birth control pill use. So, there are the common blood clots. Then there were the blood clots, these very, very, very rare blood clots that could be caused by the Johnson & Johnson vaccine.

Are there any differences between these two types of clots? And the answer is, yes. There are two major differences. The blood clots that are caused by this J&J vaccine are associated with something called thrombocytopenia, which is a lowering of the platelet count. Platelets are those cells in your bloodstream that help the blood clot. So that's odd. It's odd that you would actually have a lowering of the platelet count, which is usually associated with bleeding, not clotting. But that is true. It's called thrombotic thrombocytopenia. So, it's not the typical blood clots.

The second thing is the mechanism by which the Johnson & Johnson vaccine is causing those blood clots. It causes it by actually activating something called platelet factor 4, which is a protein that's made by platelets that helps platelets aggregate, or said another way, helps platelets form a clot. So, in the people that have these blood clots, they have two things that normally people who have blood clotting don't have. They have a lowering of the platelet count and they also have antibodies in their bloodstream directed against platelet factor 4. So that's the mechanism by which these two different types of clots are very different.

But again, I want to underscore how rare this kind of blood clotting is. As of interest that when people get infected with SARS-CoV-2, when they get the disease COVID 19, they also can get clots. In fact, about 16% of people, roughly 1 out of every 7 people, who get infected with SARS-CoV-2 will have problems with increased blood clotting, and they can have the serious kinds of blood clots, including the blood clots in the brain, which occur in roughly 5 to 6 per million people, which again is much greater than is seen in the roughly 2 per million that is seen in blood clotting associated with the Johnson & Johnson vaccine.

So, because this virus is common and because it's important to prevent, it is still important obviously to get all these vaccines. But this rare blood clotting phenomenon associated with the J&J vaccine is likely real and, therefore, should be looked for. When, for example, you've gotten the J&J vaccine and you have things like headache or difficulty breathing or leg pain and you feel it's serious enough to see a doctor, it’s very simple to tell whether or not you have a blood clot that's caused by this vaccine, which you just get something called a CBC, which just stands for complete blood count. It's available 24 hours a day in any hospital. And if you have a normal platelet count, then that is not anything that was caused by this vaccine.

Thank you very much.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Jul 14, 2021