The Link Between J&J’s COVID Vaccine and Blood Clots: What You Need to Know

A blood clot is what it sounds like—a gel-like clump of blood. Small blood clots form whenever you cut or scrape your skin to plug up the injured area and stop the bleeding. But they are dangerous when they form within a blood vessel, where they can cause thrombosis (meaning they block blood flow), a condition that kills up to 100,000 people a year in the United States. “Those clots typically occur in people who are bedridden or hospitalized, or have other medical issues related to inflammation or infection or cancer,” says Yale Medicine hematologist Robert Bona, MD. They are also more likely to occur in women who are pregnant or on oral contraceptives, or in people who have hereditary disorders that predispose them to blood clotting.

As mentioned above, the clotting condition associated with the J&J vaccine is called thrombosis with thrombocytopenia (TTS). Thrombosis occurs when blood clots block veins or arteries, and thrombocytopenia is a condition characterized by abnormally low platelet levels in the blood. The latter is unusual in someone with a major blood clot because platelets are colorless blood cells that have the function of helping blood clot.

“The mechanism of the action of these clots has been quite unusual and, frankly, that has surprised me,” says Dr. Bona. “There are definitely a lot of questions.”

A small number of serious blood clots has also been reported in people who received the AstraZeneca vaccine, which is not authorized for use in the United States. In March 2021, countries in Europe and elsewhere put a pause on that vaccine after a handful of people—mostly women younger than 60—also developed TTS. The European Medicines Agency (EMA) investigated the situation and concluded that these complications should be listed as very rare side effects of the AstraZeneca vaccine, and said the benefits still outweighed the risks. But several countries have restricted use of the vaccine because of the clots.

Both the AstraZeneca and Johnson & Johnson vaccines, as well as the Sputnik V vaccine in Russia (also not approved for use in the U.S.) are carrier—or vector—vaccines, which instruct human cells to make the SARS CoV-2 spike protein. For this vaccine technology, scientists engineer a harmless inactivated common adenovirus (which can cause colds and other illnesses when it is active) into a sort of Trojan Horse that carries genetic code to a vaccine recipient’s cells. The code then instructs the cells to produce a spike protein that trains the body’s immune system, which then creates antibodies and memory cells to protect against an actual SARS-CoV-2 infection.

The Pfizer-BioNTech and Moderna COVID-19 vaccines, both of which are mRNA vaccines and use a different method to protect against the virus, have not been linked to blood clots at this time.

Researchers have also seen a strong association between blood clots and COVID-19 infection itself, says Hyung Chun, MD, a Yale Medicine cardiologist. “Unfortunately, in those who are sick enough with COVID-19 to be in the intensive care unit [ICU], blood clots have been a major factor in their illness,” Dr. Chun says. Close to 20% of COVID-19 patients in the ICU develop blood clots, he says. “That’s far higher than what you’d expect for patients who are in the ICU for different conditions. Estimates I’ve seen are in the 3 to 10% range for patients admitted for other reasons," adds Dr. Chun.

COVID-19 infection seems to cause the blood vessels and the blood itself to behave in a way that promotes formation of blood clots, which is likely a key driver for poor outcomes, sometimes damaging vital organs and even leading to death, Dr. Chun says.

Dr. Chun and colleagues published a study in The Lancet Haematology after identifying a leading mechanism behind blood clots in COVID-19 patients. Yale Medicine hematologist Alfred Lee, MD, PhD, and George Goshua, MD, a hematology-oncology fellow, were also authors of the study. They found that endothelial cells (cells that line the blood vessels) play a surprising role in the formation of blood clots, especially as a COVID-19 patient becomes critically ill. They hope the finding will eventually contribute to determining treatment for the blood clots. So far, “there is not a clear-cut answer,” Dr. Chun says.

While the risk for a rare blood clot is low, anyone who still gets the Johnson & Johnson vaccine should know about early symptoms of the disorder. The CDC recommends seeking immediate medical care if you develop any of the following symptoms after getting the shot, especially if you are a woman aged 50 or younger:

• Severe or persistent headaches or blurred vision
• Shortness of breath
• Chest pain
• Leg swelling
• Persistent abdominal pain
• Easy bruising or tiny blood spots under the skin beyond the injection site

Anyone who experiences signs of a blood clot or any adverse reaction to a vaccine should contact VAERS.

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Note: Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.