A medical technician holding a dose of the Johnson &amp; Johnson covid-19 vaccine.

A medical technician holding a dose of the Johnson & Johnson covid-19 vaccine.
Photo: Michael Ciaglo (Getty Images)

Health regulators in the U.S. are calling for the Johnson & Johnson covid-19 vaccine to be temporarily suspended from use, following a small number of reports of a possible link between the vaccine and severe blood clotting. The potential added risk appears to be very rare, and no such connection has been documented for the Moderna and Pfizer/BioNTech vaccines.

Early Tuesday morning, the New York Times reported on the joint decision by the Food and Drug Administration and the Centers for Disease Control and Prevention to call for the J&J vaccine pause. Soon after, both agencies released a statement explaining the rationale for their decision.

According to the health agencies, there have been six cases of this type of blood clotting out of the 6.8 million Americans who have received the one-dose vaccine so far. In these cases, which all involved women between the ages of 18 and 48, people developed cerebral venous sinus thrombosis (CVST), which is when a blood clot forms in the venous sinuses, the passages that allow blood to be drained from the brain. These patients also had low blood platelet levels, a condition called thrombocytopenia. Symptoms began to develop within six to 13 days after taking the vaccine.

The reports have prompted the agencies to investigate the link between CSVT and the vaccine, which will include a meeting by the CDC’s independent panel of experts, the Advisory Committee on Immunization Practices (ACIP). But until that investigation is complete, they’re “recommending a pause in the use of this vaccine out of an abundance of caution,” their statement said. At the same time, they emphasized that the possible risk of CVST seems to be extremely small.

Saskia Popescu, a public health researcher and infectious disease epidemiologist at George Mason University, said that this decision shouldn’t be seen as a death knell for the vaccine.

“This doesn’t impact the efficacy of the vaccine, and it’s important that we also communicate effectively that this is not a permanent pause and an abundance of caution is how the FDA functions with any safety concern to ensure transparency, trust, and safety,” she told Gizmodo in an email.

CVST is a rare, treatable, but life-threatening form of stroke. Oftentimes, these clots can cause cells to burst and leak blood into the brain, leading to a brain hemorrhage. Normally, it’s thought to affect five out of every 1 million people a year, most commonly infants in the first month of life. Symptoms include headache, blurred vision, loss of movement control, and seizures.

In March, reports of CVST following the use of the AstraZeneca vaccine emerged in several European countries, which similarly involved younger people. The reports prompted many countries to temporarily suspend their campaigns. The European Union’s regulators have since concluded that the vaccine’s benefits overwhelmingly outweighed its risks, but that there might be a small added risk of CVST from its use and that it should be included in a list of potential side effects. Some countries have resumed the use of the AstraZeneca vaccine but are currently limiting it to people 55 or older.

There are various theories for how these vaccines could be causing CVST or other rare forms of blood clotting. Because covid-19 itself is often linked to clotting issues, one theory is that vaccination in general can rarely cause a similar immune response that leads to CVST. However, no clotting issues have been spotted with either the Pfizer or Moderna mRNA vaccines, throwing doubt on that theory. Some other scientists have speculated that the neutered adenovirus used in the AstraZeneca vaccine is the primary culprit, inadvertently triggering a unique and rare immune response separate from the response seen with natural covid-19. And since Johnson & Johnson’s vaccine also works by using a neutered adenovirus as the delivery method, that could lend credibility to this theory.

Understanding how these vaccines could be linked to CVST is important. But Angela Rasmussen, a virologist affiliated with Georgetown University, notes that other common medications carry a much higher risk of blood clots and are still allowed on the market since their benefits outweigh these risks, such as birth control pills. Even if the rare risk of CVST from the vaccine is confirmed, she adds, that shouldn’t necessarily deter people from taking it nor worry people who have already taken it, given that the risks of complications from covid-19, including blood clots, are so much higher than from the vaccine.

“I got the J&J vaccine eight days ago. I’m a woman between the ages of 18-48. I am not losing sleep over this, because the chances of me having this complication—if it is linked to the vaccine—are overwhelmingly slim,” she said in an email. “I hope others who got J&J will feel reassured that this is a sign that the FDA takes safety extremely seriously, but it doesn’t mean they are at high risk.”

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