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Doctors & Advice, Family Health

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

BY KATHY KATELLA December 17, 2021

New data prompts the CDC to update their advice.  

[Originally published: April 21, 2021. Updated: Dec. 17, 2021.] 

Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.

Concern has been building over a small, but growing number of cases of a rare, but serious blood clotting disorder associated with the Johnson & Johnson coronavirus vaccine. As a result, in mid-December, the Centers for Disease Control and Prevention (CDC) endorsed their advisory committee’s unamimous decision to give a preferential recommendation to the Pfizer-BioNTech and Moderna vaccines.  

The clotting disorder is called thrombosis with thrombocytopenia syndrome (TTS), and it is rare—in 17 million people who got J&J, 54 cases of TTS were reported, 36 of those people were cared for in an intensive care unit, and nine people died (which represents over 15% of cases). Two additional deaths also may be linked to the vaccine. Based on these numbers, the risk appears to be greatest—1 in 100,000—in women ages 30 to 49. This updated CDC recommendation follows similar recommendations from other countries, including Canada and the United Kingdom. 

The J&J vaccine is still available. The FDA, which has updated its fact sheet on the shot, still says that the risks of the virus are greater than the risks of the J&J vaccine. If you got the J&J shot, your risk of TTS would be expected to be low. In terms of timing, all of these cases were identified within two weeks of the person receiving the vaccine.

While there are three vaccines authorized for use in the United States, there was hope that Johnson & Johnson’s would be an important one, partly because experts believed its one-shot dose could help with those who didn’t have access to mRNA vaccines. But blood clots became a concern starting in April, when the government put a pause on the J&J shot after six women who received it developed rare blood clots—and one died. All of these cases were reported to the Vaccine Adverse Events Reporting System (VAERS), a national early reporting warning system to detect safety problems with U.S.-licensed vaccines.

The new CDC recommendation follows similar recommendations from other countries, including Canada and the United Kingdom.

In April, Yale Medicine infectious diseases specialist Jaimie Meyer, MD, MS, said the pause on the Johnson & Johnson vaccine was not an unusual step. “Even after it has completed its clinical trials, a vaccine still must be monitored in the real world,” she said. “If there are serious adverse events, we need to stop, take a deep breath, collect and analyze the data—and really try to understand why this is happening. Then, we can decide whether we should proceed.”

Transparency is also important, Dr. Meyer added. “People should know that there are these events that have been observed—even if they are very rare events—so they can make informed choices.” 

What’s troubling about the reported clots

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.”

We saw this before—with the AstraZeneca vaccine

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, 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.

Blood clots are also a complication of COVID-19

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.  

TTS symptoms to look for

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

As of December 2021, about 16 million people in the United States had gotten Johnson & Johnson’s single shot compared to 73 million who got Moderna’s two-dose primary inoculations and 114 million who got Pfizer. A mix-and-match strategy allows people to choose any vaccine they want for a booster shot. Only 1.6% of people in the U.S. have chosen a J&J booster. The CDC updates these numbers regularly in its COVID Data Tracker.

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

[Visit Yale Medicine's Vaccine Content Center for more stories on COVID-19 vaccines.]

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.