The U.S. Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) have suggested a pause in the use of the Johnson & Johnson (J&J) COVID vaccine after receiving reports of a rare blood clot disorder, according to Medscape Medical News.

After reviewing data, the CDC and FDA found six people who were given the J&J vaccine and experienced rare blood clots in combination with low platelets, prompting the agencies to initiate the pause “out of an abundance of caution.”

In a joint statement, the agencies said that all six cases were in women between the ages 18 and 48. They reported symptoms within 6 to 13 days of vaccination. The women had an extremely rare type of blood clot in the brain called cerebral venous sinus thrombosis. Plus, the condition was seen in combination with low levels of blood platelets.

Of those six cases, one woman died, while another was in critical condition.

The CDC and FDA said that the treatment for this type of blood clot is different from that for other types. They said an anticoagulant drug, heparin, is often used in other types of blood clot disorders; however, for this type of rare clot, the patients need an alternative treatment.

So, why did it happen?

Dr. Robert Glatter of Lenox Hill Hospital in New York City said, “We don’t have a definitive answer at this time, but it appears to involve an immune response related to the J&J vaccine that adversely affects the function of the platelets, which in turn prevents the normal clotting process from occurring.”

He said that the explanation “may ultimately be related to the adenovirus vector [used in the vaccine] itself.”

It is also unclear why did it happen to women than men. Infectious disease expert Dr. William Schaffner of Vanderbilt University Medical Center in Nashville said, “This has been true for the AstraZeneca [vaccine] as well and their blood clot issues. If it is an immune-related issue, women do tend to have more of those issues than men, in general.”

For people who received the J&J vaccine, Dr. Schaffner suggested that the reports of blood clots were rare, adding that only one in a million may have it.

“This is actually a testament to the sensitivity of our vaccine safety system,” he explained, that those few reports would be detected out of millions of doses, according to Medscape.

The CDC and FDA said, “People who have gotten the vaccine should be vigilant of symptoms. If you received the Johnson & Johnson vaccine and develop a severe headache, abdominal pain, leg pain, or shortness of breath within 3 weeks of vaccination, contact your health care provider.”

The CDC’s Advisory Committee on Immunization Practices (ACIP) will meet today to further review the cases and “assess their potential significance.” The article was published on Medscape.