The Centers for Disease Control and Prevention, along with the Food and Drug Administration, lifted the 11-day pause on Friday of the Johnson & Johnson one-shot COVID-19. The officials put a temporary halt on the vaccine distribution after women who took the shot developed blood clots. Three of them died.

According to CNN, out of nearly 8 million doses of the vaccine, there were 15 cases of thrombosis with thrombocytopenia syndrome, or blood clots with low platelets. Dr. Leana Wen, an emergency physician who took part in the J & J clinical trials, said that while TTS is very rare, it appears that the syndrome caused by the vaccine occurs in younger women. Wen, who got the placebo during the trials but elected to get the J & J “one-and done” vaccine, said that it is important for women to consider the alternative two-dose vaccines made by Pfizer and Moderna that appear not to carry the risk of clotting.

While 15 cases out of 8 million disease does not appear to be risky, the Centers of Disease Control evaluated the risk by focusing on a specific population of women under the age of 50. By narrowing the cohort, the risk of TTS from the vaccine now becomes 1 in 80,000, says Dr. Wen.

“The risk is still low and is certainly much lower than the risk of contracting coronavirus and having a severe outcome, but we have to stop saying things like the risk of TTS is the same as the lifetime risk of being struck by lightning,” she said. “That may be true for the overall population but does not appear to be true for the subgroup of women under 50.”

Wen said that experts could not find specific common reasons why these women were affected but given the rarity of TTS and that fact these women developed the condition after getting the vaccine suggests a link, the told CNN.

She added that TTS was also observed in women taking the AstraZeneca vaccine which shares the same technology as the J & J shot.

Paul Offit, an infectious disease expert and the director of the Vaccine Education Center at The Children’s Hospital in Philadelphia, said that the FDA carefully looked into cases of thrombosis during the approval process of the J & J vaccine.

Since both the J &J and AstraZeneca vaccines used 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,” said Offit, according to STAT. “So that will be determined, I suspect soon.”

Wen says of  women who get the warning signs of TTs, including a severe, unrelenting headache, trouble breathing and chest pain, abdominal pain, leg swelling, sudden numbness, or weakness in an arm of leg and easy bruising: “Call your doctor immediately if you have any of these symptoms,” she said, per CNN.

Besides the three women who died of TTS after getting the J & J vaccine, seven are still in the hospital, and four are in critical condition. The largest cluster of sufferers in terms of age was in the mid-to-late 30’s with a median age of 37.