MILWAUKEE - Use of the Johnson & Johnson COVID-19 vaccine remains paused amid an investigation into rare but serious reports of blood clots among some who received the shot.
So far, the majority of cases have involved young-to-middle-aged women, who may be at greater risk.
The medical community is racing to understand why six people developed blood clots in their brains after receiving the Johnson & Johnson vaccine.
"We want to make sure these vaccines number one, that they’re safe themselves, but is there a population of people that are safest to receive this vaccine as well?" said Dr. William Hartman, principal investigator of the UW Health AstraZeneca COVID-19 vaccine trial.
The patients were all women between the ages of 18 and 49. A 25-year-old man suffered the same condition during clinical trials in 2020. Hartman said that's likely not a coincidence.
"Women may be more susceptible to forming these blood clots -- any side effect really that has to do with vaccines -- because the cells that produce these antibodies, they’ve got estrogen receptors on them," Hartman said.
The Johnson & Johnson vaccine uses similar technology as the AstraZeneca vaccine. No clotting cases have been observed in the U.S. AstraZeneca vaccine trial, but in Europe, where AstraZeneca is already approved for use, several cases have been identified.
Still, Hartman urges women -- and men -- not to panic. He said the likelihood of developing a blood clot in the brain from the vaccine is one in one million. He said people are at 10-times greater risk of developing the same kind of clot from a COVID-19 infection.
"That protection you get from the vaccine, it still outweighs that risk of developing COVID and getting the blood clots," said Hartman.
Hartman said doctors and researchers will be looking to find any common denominators among the Johnson & Johnson patients. Some have theorized birth control pills may play a role. Data show only one of the women was taking oral contraceptives at the time.