Federal health officials urged vaccination centers across the country on Tuesday to pause their use of the Johnson & Johnson COVID-19 vaccine after six women developed blood clots within two weeks of vaccination.
The U.S. Food and Drug Administration and Centers for Disease Control and Prevention have said the condition, called cerebral venous sinus thrombosis (CVST), is “extremely rare” after getting the Johnson & Johnson shot.
The pause, which is expected to last for a couple of days, stemmed from an abundance of caution, giving officials time to understand what’s causing the severe type of clotting and prepare health care systems to recognize and treat patients appropriately if more cases appear.
One woman died from the condition after receiving the Johnson & Johnson vaccine, and another is in critical condition, officials said.
But experts say there’s no need for widespread alarm. The Johnson & Johnson vaccine makes up less than 5% of the recorded shots in arms in the U.S. to date. As of Tuesday, more than 7.2 million Americans have received the single-dose shot.
So what exactly is CVST and how common is it outside of the context of COVID-19 vaccination? Here’s what to know.
What is cerebral venous sinus thrombosis and its symptoms?
The condition occurs when a blood clot forms in the brain, preventing blood from draining out of it and causing blood cells to break and leak into brain tissues. Medically, this is known as a cerebral hemorrhage.
“It is like water in a reservoir overflowing into the surroundings or like a ruptured dam,” according to the University of Michigan Comprehensive Stroke Center.
The condition is essentially a rare form of stroke that causes symptoms such as numbness, weakness in the arm or legs, trouble speaking, blurred vision, shortness of breath, leg pain, severe headache and abdominal pain.
Medications that contain estrogen such as birth control pills, pregnancy, inherited blood clotting disorders and drugs for chemotherapy are some of the most common causes of CVST.
However, there does not seem to be a link between birth control use and the six cases reported in the U.S., all of which occurred in women between ages 18 and 48, Dr. Peter Marks, director of the Center for Biologics Evaluation and Research at the FDA, said during a news conference Tuesday morning.
The condition is typically diagnosed with an MRI or CT scan.
How common is CVST?
CVST typically occurs in two to 14 people per 1 million, Marks said. However, what’s “notable” is that the six cases reported in the U.S. after Johnson & Johnson vaccination were coupled with low blood platelet counts, which together “make a pattern” that’s even more rare.
It’s so rare that there is no annual rate of occurrence that health care professionals are aware of.
Generally, CVST is more common among women, particularly those between the ages of 20 and 35, because of pregnancy and oral contraceptive use, according to a review published in 2000 in The BMJ, a journal published by the British Medical Association. Yet, studies show the mean age of those affected is between 37 and 38 years old.
Although CVST is uncommon, diagnoses have been occurring more frequently because of a “greater awareness and the availability of better non-invasive diagnostic techniques,” the review said. “The clinical spectrum, however, is wide and recognition remains a challenge for the clinician.”
There are about 5,000 CVST cases per year in the U.S., the University of Michigan notes. That makes it much more rare than other types of blood-clotting conditions, such as deep vein thrombosis and pulmonary embolism, which has 1 million cases per year.
About 80% of patients with CVST recover without any functional disabilities, according to Neurology Live. “(N)evertheless, 5% to 10% of patients die in the acute phase.”
How is CVST treated?
Typically, blood clots are treated with an anticoagulant called Heparin, but the medication “may be dangerous [for CVST], and alternative treatments need to be given, preferably under the guidance of physicians experienced in the treatment of blood clots,” Marks said during the news conference.
Usual treatment would usually last about three to six months if the blood clot trigger was temporary, such as pregnancy, but if the cause is not identified or is more severe, treatment may last six to 12 months.
The CDC said its Advisory Committee on Immunization Practices is meeting Wednesday to “review these cases and assess their potential significance.”