What is up with vaccine-induced immune thrombotic thrombocytopenia (VITT) and those weird blood clots?


A: This week, the US Centers for Disease Control and Prevention (CDC) put a “pause” on the Johnson & Johnson vaccine to investigate reports of rare blood clots potentially caused by VITT.

These reports are like the cases seen in Europe after getting the AstraZeneca COVID-19 vaccine. VITT is a super rare medical condition where a person develops low platelets and blood clots after vaccination. These folks need a special kind of treatment. Read below for more info on the condition and how it might work.

First things first: These events are exceedingly rare and the risk of severe illness from COVID-19 (including blood clots) is much higher! There is NO link to VITT with the mRNA vaccines (like Pfizer-BioNTech or Moderna).

Next, let’s go over definitions because there are some cool vocab words in here. Thrombotic means forming blood clots and thrombocytopenia means low platelets. Platelets are cells in the blood that clump up together to form blood clots. Blood clots are an important way your body stops bleeding, like if you get cut or hurt. If platelet numbers are too low, people can have dangerous bleeding.

So, what is vaccine-induced immune thrombotic thrombocytopenia?

This is all new and we are still learning more about this all the time. VITT seems to be like a different disease, called heparin-induced thrombocytopenia (HIT). In HIT, heparin (a medicine used to prevent and treat blood clots) attaches to a molecule called platelet factor 4 (PF4). PF4 and heparin stick together. Some people have antibodies that recognize this new complex as weird and bad, and the antibody sticks to the PF4-heparin blob. Receptors on platelets hold on to that blob (that now has the antibody, PF4, and heparin) and this “activates” the platelet: telling it to release its chemicals and begin to form a clot. Some platelets get used up making the clots and others get removed by the spleen, causing low platelets. The body is now in an unusual state, both forming blood clots and at risk of bleeding from low platelets.

For VITT, people didn’t get heparin. So, what is happening? Well, we don’t know exactly, but there may be a component of the J&J and AstraZeneca vaccines that might be triggering a similar response from the body. These rare cases of blood clots with low platelets have been reported after both J&J vaccines in the US and AstraZeneca vaccines in Europe. In the US, there were 6 women who developed cerebral venous sinus thrombosis (CVST) with thrombocytopenia. Five of the six tested positive for the PF4-HIT antibody (the last one wasn’t tested for this), suggesting a similar mechanism to HIT.

Don’t these folks have blood clots in strange places?

Yep, with cerebral venous sinus thrombosis (CVST) with thrombocytopenia being most common. CVST is a rare blood clot that forms in the venous sinuses of the brain. The venous sinuses are veins that drain blood from the brain back toward the heart. The blood clot can block this drainage, cause bleeding into the brain (hemorrhage) and a stroke.

What are symptoms of VITT and CVST?

People who have severe headaches, stomach pain, nausea, vomiting, vision change, difficulty breathing, or leg pain and swelling should be evaluated by their clinician. Many people have symptoms after vaccination that are NOT indicators of VITT and are super common, like fever, fatigue, mild headache and muscle aches. These usually get better in 1-2 days.

How is VITT diagnosed and treated?

Clinicians will look for blood clots by examining the individual and may order blood and imaging tests. If a blood clot is found, the @CDC recommends that the clinician look for PF4-HIT antibodies. If that is found, the person is treated with an antibody treatment given through the veins (IVIG). This treatment can help keep the body’s immune system from attacking itself. The blood clot is treated with anticoagulants (blood thinners). The important thing is that heparin is NOT used as the blood thinner. Other blood thinners are widely available.

Want to learn more? Here are some good reads:

The @AmericanSocietyofHematology VITT FAQs

The @CDC Health Alert on CVST with Thrombocytopenia after J&J Vaccination

The @NewEnglandJournalofMedicine publication on Thrombotic Thrombocytopenia after AstraZeneca vaccination

The AAFP @familymed vaccine update on J&J

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