What to Know About the Rare Blood Clots Linked to J&J’s Vaccine

The Johnson & Johnson Covid-19 vaccine is currently on pause in the United States because of concerns about blood clots. Here's what doctors need you to know.

Covid-19 vaccine side effects

Covid-19 vaccines are on everyone’s mind. While millions of people have been safely vaccinated, experts are monitoring the rollout and still concerned about avoiding side effects.

There are three Covid-19 vaccines currently available in the United States, and one, which is made by Johnson & Johnson, is currently on pause due to a possible risk of rare and potentially dangerous blood clots.

There have been six reported cases of serious blood clots in people given the vaccine, according to the Centers for Disease Control and Prevention (CDC) and Food and Drug Administration.

While the news is worrisome, the CDC and other vaccine experts stress that the risk appears to be vanishingly small. The CDC indicated that it paused administration of the Johnson & Johnson vaccine “out of an abundance of caution.”

Here’s what you need to know about the vaccine, blood clots, and risk. (If you already got the Johnson & Johnson vaccine, check out what doctors recommend you do.)

Why is the Johnson & Johnson vaccine different?

The two other vaccines available in the United States are made by Pfizer-BioNTech and Moderna, and they work in a different way than the Johnson & Johnson vaccine. They rely on genetic material called mRNA to prime the body to fight Covid-19.

The mRNA delivers instructions to immune cells that help them recognize SARS-CoV-2 (the Covid-19 coronavirus) and generate antibodies to stop the infection.

Some people have had anaphylactic or allergic reactions to the Pfizer and Moderna shots, but all recovered quickly.

Common side effects from all of the Covid-19 vaccines have mainly been a sore arm and mild flu-like symptoms.

The Johnson & Johnson vaccine uses an inactivated virus to prompt immune cells to recognize the spike protein found on the virus that causes Covid-19. The immune system then creates antibodies to fight the virus.

Blood clots and the Johnson & Johnson vaccine

So far, more than 6.8 million people in the United States received the Johnson & Johnson vaccine. Concerns about the vaccine emerged after six women between the ages of 18 and 48 developed rare blood clots. The symptoms began six to 13 days after vaccination. One of the women died, and another is in critical care.

The reason for the clots appears to be an interaction between antibodies made by the body and platelets—blood cells that help with clotting, according to Jack Ansell, MD, former chairman of medicine, Lenox Hill Hospital, and professor of medicine at Hofstra-Northwell School of Medicine.

The antibodies cause the platelets to clump together, forming a blood clot known as cerebral venous sinus thrombosis (CVST). The clots can occur in the brain and prevent blood from draining out. There can also be hemorrhaging in the brain, leading to a stroke or stroke-like symptoms. (What to know about Covid-19 and stroke risk.)

This phenomenon has a name due to a similar clotting risk already established with a vaccine made by AstraZeneca, which is not available in the United States, explains Stephan Moll, MD, professor of medicine at the University of North Carolina School of Medicine, Chapel Hill: vaccine-induced prothrombotic immune thrombocytopenia (VIPIT).

A small number of people have developed rare blood clots with the AstraZeneca vaccine shot. The exact number is unclear, but according to studies in the New England Journal of Medicine, there have been five cases out of 130,000 vaccinations. It also seems to occur more often in younger people, and primarily women.

(Don’t fall for these 19 coronavirus myths.)

In this photo illustration a Johnson & Johnson logo is seen...SOPA Images/Getty Images

What to know about the risk

The first thing to remember is that no one is certain the vaccine is responsible, explains Dr. Moll: “At this point, it is not known whether the clots are causatively related to the vaccine or coincidental,” he says. “Six reported cases do not make an epidemiologic study.”

Even if a connection is established, the risk seems to be extremely low, says Dr. Ansell. “So far, we have six cases from almost seven million vaccines administered in the United States with Johnson & Johnson,” he says. Those odds are less than one in a million. “On the other hand, getting Covid is a fairly likely event and can be much more harmful and fatal.”

Purvi Parikh, MD, an immunologist with Allergy & Asthma Network adds that the risk of blood clots with the Johnson & Johnson vaccine is lower than other causes and risk factors for blood clots, such as an actual Covid-19 infection, taking birth control, obesity, smoking, and cancer.

Even everyday activities can be risky to your health in general, like driving, swimming, bicycling, or walking in the rain. Of course, we take measures to minimize those risks, whether it’s swimming lessons, wearing a bike helmet, or staying indoors in inclement weather.

You can do the same with the vaccine by learning blood clot symptoms and monitoring your health in the few weeks post-vaccination when the risk seems to be present.

The symptoms of note include a severe headache that doesn’t subside, significant abdominal or leg pain that also does not subside, or increasing shortness of breath. If you experience these within two to three weeks of having had Johnson & Johnson’s vaccine, notify your health care provider.

But after about three weeks, says Dr. Ansell, the research suggests that the risk period for these types of rare blood clots is over.

Would experts get the vaccine?

“If I had the choice, I would get the Pfizer or Moderna vaccine as there has been no signal of increased blood clots, however rare they appear to be,” Dr. Moll says. “However, if only the Johnson & Johnson vaccine was available, I would definitely get that rather than to decide not to get vaccinated at all, and would not worry about a blood clot, as the blood clot risk appears to be so very low.”

“My daughter just received the Johson & Johnson vaccine two weeks ago,” says Dr. Ansell, who believes the blood clot risk shouldn’t stop people from getting the vaccine. “She called me and wanted to know more and I told her the risk is almost zero.”

The pause in administering the vaccine makes sense, Dr. Ansell says.

“I think that now that the CDC and FDA paused the administering this vaccine for a few days to just evaluate these cases to learn more information is certainly appropriate at this time. But when the information is found out, and if it’s nothing new or more than what I described, I suspect that the vaccine will still be in use.”

“I would get the J & J vaccine,” says Dr. Ansell.  “I would not specifically recommend one vaccine above another.  They are all very effective in preventing serious illness, hospitalization, and death. Individuals should feel safe and comfortable with whichever vaccine is being used at a particular site.”

“The Johnson & Johnson vaccine is not very risky,” Dr. Parikh says. “In general, the benefits of the vaccine still far outweigh the risks.”


Emily DiNuzzo
Emily DiNuzzo is the former associate editor at The Healthy and a former assistant staff writer at Reader's Digest. Her work has appeared online at the Food Network and Well + Good and in print at Westchester Magazine, and more. When she's not writing about food and health with a cuppa by her side, you can find her lifting heavy things at the gym, listening to murder mystery podcasts, and liking one too many astrology memes.