6 Women Developed This Rare Blood Clot After Receiving the Johnson & Johnson Vaccine-Here's What to Know

Federal health officials on Tuesday called for a pause in the use of Johnson & Johnson's (J&J's) one-dose COVID-19 vaccine following six reports of a rare type of blood clot known as a cerebral venous sinus thrombosis, or CVST, in women who received the vaccine. One woman died and another has been hospitalized in critical condition, according to news reports.

In a joint statement issued Tuesday, the US Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) recommended a temporary standstill in vaccine administration "out of an abundance of caution." All six cases were in women ages 18 to 48, and occurred in combination with low levels of blood platelets, a condition known as thrombocytopenia, the agencies noted.

The FDA and CDC intend to further investigate these cases and assess their significance. FDA Acting Commissioner Janet Woodcock, MD, told reporters on Tuesday that such adverse events appear to be "extremely rare." Nevertheless, she stressed that COVID-19 vaccination safety is a top priority and that the administration takes all such reports very seriously.

In part, the pause is meant to ensure that heath care providers are aware of the potential for this type of blood clot to occur. Again, as a precaution, the agencies want providers to be prepared to recognize, manage, and treat vaccine recipients who experience this adverse event.

At this point, it's not known whether the J&J vaccine caused the clots. In a prepared statement, J&J noted that the government is reviewing data on six cases out of 6.8 million doses administered. The vaccine maker also noted that it is reviewing the cases with European health authorities and had decided to delay the vaccine rollout in Europe.

If you recently had a J&J shot, here's what experts want you to know about this rare clotting complication.

What is cerebral venous sinus thrombosis, or CVST?

A CVST is not like the type of blood clot that blocks blood flow to the brain. This type of clot forms in the venous sinuses, which are spaces in the skull that allow blood to drain from the brain. (FYI, they're not the same sinuses that fill with mucus when you get a cold. That's a separate drainage system.)

When a blood clot forms in the venous sinuses, it's like having a clog in your plumbing. "This is basically preventing blood from draining out of the brain," Stacey Rose, MD, assistant professor of internal medicine and infectious diseases at Baylor College of Medicine, tells Health. And, she says, that congestion of blood in the brain is what leads to stroke—a very rare type of stroke affecting fewer than five in a million people every year, per Johns Hopkins Medicine.

James Bussel, MD, professor emeritus of pediatrics at Weill Cornell Medicine in New York City, who studies blood disorders, explains the chain of events this way: Your platelets, which are blood cells that help your blood to clot, somehow get activated; a clot forms in these large sinuses; and you end up with a low platelet count because you're using up platelets. That, in turn, can lead to bleeding, he tells Health.

What are the CVST symptoms to watch out for?

The FDA and CDC advise people who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after getting their shot to contact their health care provider. (Belly pain can be a sign that the person has developed clots in the abdomen, Dr. Bussel notes.)

In the six reported cases, symptoms appeared six to 13 days after vaccination. "Clearly we need to get a diagnosis as soon as possible in patients who are having these sorts of symptoms," Dr. Rose observes.

However, a common blood-thinning medication called heparin, often used to treat blood clots, may, in these instances, make the problem worse, Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, pointed out during a White House press briefing on Tuesday.

As Dr. Rose explains, "It might be that if you've already had some sort of backup into the brain, you wouldn't want to give heparin because then it's just going to sit in the blood in the brain and cause more damage."

The takeaway: This shouldn’t make you shy away from getting vaccinated

It's impossible to know at this moment why these adverse reactions occurred, especially because these events appear to be extremely rare (literally less than one in a million, if you figure that only six women among 6.8 million people who have received the J&J experienced CVST).

Studies of CVST in general highlight certain risk factors. "For whatever reason, women in the postpartum period are at higher risk," Dr. Rose points out. Perhaps that has something to do with hormones, she says. Whether this was the case for the six women who received the J&J vaccine is unknown, but the American College of Obstetricians and Gynecologists issued a statement on Tuesday advising their ob-gyns to steer their patients seeking COVID vaccination to one of the other two vaccines currently available: "At this time, there is no clear phenotype of women who are more or less likely to experience this rare complication," ACOG noted. "However, until there is a better understanding of the frequency and impact of this finding, it will be important to encourage pregnant and postpartum women who wish to be vaccinated to receive the mRNA vaccines: Pfizer or Moderna."

Other factors, like having a clotting disorder or an autoimmune disorder, like lupus, can also put people at risk for these blot clots, says Dr. Rose—but again, that's for CVST in general and not necessarily linked to those six women who received the J&J vaccine.

Overall, Arthur Caplan, PhD, professor of bioethics at the New York University Grossman School of Medicine in New York City, says we don't collect good baseline information from regular people (aka, those not in clinical trials) getting vaccines. So if an adverse reaction pops up, it can be hard to tell whether the vaccine caused it or if something else is going on. That's why the CDC and FDA both recommended a pause on administering the vaccine in order to investigate these cases.

And for those people who are now questioning whether to get a COVID shot, Caplan says, "Fear the virus, not the vaccine." Birth control pills, he notes, are much more likely to cause blood clots than COVID vaccines. Meantime, thousands of Americans are dying or being hospitalized every day with COVID-19—another condition linked to a higher risk of blood clots than J&J's vaccine. "You don't want to be talking about remote, tiny risks from vaccines at graveyards," he says.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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