The U.S. government has recommended a pause on the distribution of the Johnson & Johnson COVID-19 vaccine after a potentially deadly side effect was noted in six women out of the 6.8 million people given the drug. The rare side effect, called cerebral venous sinus thrombosis (CVST), occurs when clots prevent blood from draining from the brain.

According to STAT, similar side effects occurred in people who received the AstraZeneca vaccine. Researchers worldwide are trying to unravel the reason these vaccines are causing rare but serious side effects. According to The Guardian, the clotting disorder is accompanied by low platelet counts, and most cases have been CVST related. The death rate is high.

A study in Norway and a second review which examined patients in Austria and Germany, suggested that the rare clotting experienced after the COVID-19 vaccines resemble a condition seen in patients who are treated with the blood thinner heparin.

“It’s extremely convincing,” said Dr. David Juurlink, a Canadian pharmacologist and internist whose research specialty is investigating adverse drug reactions, according to STAT.

The studies so far indicate that blood clots occurred in people with high levels of antibodies to platelet factor 4. While this does not explain why the vaccines could trigger this reaction, it does give a clue about what is happening in the body and what to look for to potentially avoid it in the future, according to STAT.

“We know what’s happening, but the ‘why’ is unknown,” said Dr. Jose Perdomo, a hematology expert at the University of New South Wales, according to The Guardian.

“It’s now a recognized syndrome, a recognized disorder, and most importantly there is a combination of tests that can establish if a patient has this or not,” said Dr. Theodore Warkentin, of McMaster University in Ontario, who co-authored one of the studies.

Healthcare experts say they do not know if the COVID-19 vaccines trigger CVST or these people would have suffered the clots anyway. On Tuesday, Peter Marks, of the Food and Drug Administration, addressed reporters and said that so far, the side effects have not been seen in either the Pfizer or Moderna vaccines in the U.S., as per STAT.

However, Peter Arlett, head of pharmacovigilance and the head of the European Medicines Agency, said earlier that there were 35 cases of serious blood clots among the 54 million doses of the Pfizer vaccine administered and five cases seen among the Moderna recipients in Europe. Arlett added that there have been four cases of blood clots reported for the 4.5 million who received the Johnson & Johnson vaccine.

According to STAT, Arlett revealed that during the clinical trials of the J & J vaccine, researchers detected “an early sign” of an increase in venous thromboembolism and the EMA began an investigation of the clots. So far, no causal relationship has been found and the occurrence is extremely rare.

Juurlink said that for some patients the benefits of getting the AstraZeneca or J & J vaccines far outweigh the risks. And Paul Offit, an infectious disease expert and the director of the Vaccine Education Center at The Children’s Hospital in Philadelphia, questions why a vaccine would cause the production of antibodies against platelet factor 4. Offit said that the FDA carefully looked into cases of thrombosis during the approval process of the J & J vaccine.

Since both vaccines use modified versions of the adenovirus to teach our immune systems to ward off COVID-19, that is where the answer could lie.

“There is going to be something about the adenovirus — whether it’s adenoviral DNA or an adenovirus protein — that complexes with platelet 4,” said Offit, according to STAT. “So that will be determined, I suspect soon.”