Blood Clots and COVID-19 Vaccines: What You Need to Know

What should patients know about blood clots and COVID-19 vaccines?

They are extremely rare

First: These events are extremely rare. In the unusual event that what’s being called “vaccine-induced thrombotic thrombocytopenia (VITT)” occurs, Michigan Medicine clinicians have the tools to diagnose and treat this condition. VITT is characterized by the presence of two conditions at the same time: thrombosis, or blood clots (often in unusual sites like the brain, as opposed to the most common locations of blood clots like the legs and lungs) AND thrombocytopenia, or low platelets.

Vaccine-inducted thrombotic thrombocytopenia (VITT) has also been referred to as vaccine-induced prothrombotic immune thrombocytopenia (VIPIT). This condition is similar to heparin-induced thrombocytopenia (HIT) but is associated with prior administration of the Johnson & Johnson/Janssen or AstraZeneca COVID-19 vaccines without prior exposure to heparin.

The CDC announced 6 cases among more than 6 million people vaccinated with Johnson & Johnson/Janssen COVID-19 vaccine. Ongoing investigations will help to better describe the incidence of VITT and inform the CDC’s future decision on vaccine administration based on overall benefits and risk.

They have only been seen with the J&J and AstraZeneca COVID-19 vaccines

Second: VITT has only been seen with the Johnson & Johnson/Janssen and AstraZeneca COVID-19 vaccines (NOT Moderna or Pfizer, the two vaccines Michigan Medicine is currently administering). AstraZeneca’s vaccine is not available in the United States and the Johnson & Johnson/Janssen vaccine program has been placed on hold.

Overall, the available COVID-19 vaccines are very safe and highly effective at preventing COVID-19 infection. Michigan Medicine encourages all patients to get their first available COVID-19 vaccine as soon as possible, given the ongoing pandemic and known risks of COVID-19 infection.  Finally, the apparent risk of VITT with a COVID-19 vaccine is greatly outweighed by the risk of complications from COVID-19 infection (including hospitalization, blood clots and death).

The timeframe of risk is narrow — 4-28 days after vaccination

Third: If you did receive the Johnson & Johnson/Janssen vaccine, the presumed window of risk is quite narrow – between 4 and 28 days after vaccination. VITT has not been reported to occur immediately (within 1-2 days) or longer-term (beyond 3-4 weeks) after vaccination.

Demographic and clinical risk factors for the development of VITT are uncertain. Most patients who developed VITT were younger than 60 years old. While all of the cases following the Johnson and Johnson/Janssen vaccine in the United States occurred in women, both men and women have been diagnosed with VITT following the AstraZeneca vaccine in other parts of the world. There is no evidence that patients with a history of thrombosis, thrombophilia, or HIT are at increased risk for VITT.

Symptoms to watch for

Between 4 and 28 days after your J&J/Janssen vaccine, watch for symptoms of a clot including severe headache, vision changes, severe abdominal pain, or nausea and vomiting.

Your physician may recommend imaging appropriate to your symptoms (e.g., CT or MR venogram of the head for suspected cerebral sinus vein thrombosis) along with complete blood count laboratory tests.  If either test is normal (no thrombosis or thrombocytopenia), then VITT can be excluded.  If BOTH tests are abnormal (acute thrombosis and thrombocytopenia), hospitalization for further evaluation and treatment guided by a hematologist or other thrombosis expert is appropriate.

Don't use blood-thinning medications to prevent VITT

Finally: Use of any blood thinning medications to prevent VITT (e.g., aspirin) is not recommended.  These events are extremely rare and there is no evidence that any medication would prevent them from occurring.

More information

Note: The information on this page is adapted from the American Society of Hematology and American College of Cardiology FAQ co-authored by Michigan Medicine COVID Vaccine & Therapeutics Taskforce:  Safety & Efficacy Group on April 21, 2021.