CDC says women under 50 are at the highest risk of blood clots after J&J shot as advisors debate the vaccine's future

• Fifteen women have developed blood clots after receiving the J&J vaccine. Three cases were fatal.
• The US paused the shot earlier this month after six cases of blood clots were reported.
• Experts are meeting to discuss what to do with the vaccine, including attaching a warning label.

Fifteen women who received Johnson & Johnson's COVID-19 vaccine have developed a rare type of blood clot in their brains, officials disclosed in slides to be presented at a Centers for Disease Control and Prevention meeting on Friday afternoon. Three of the clots were fatal, the slides say.

Officials had previously disclosed that a 45-year-old woman in Virginia died in March after receiving the vaccine. The CDC and public-health officials have said they're investigating the death of a woman from Oregon who was in her 50s. Details of the third death have not been shared.

Seven women with the clots remain hospitalized (four of them in intensive care), while five others have been discharged.

An independent expert panel is meeting Friday to assess the risk of blood clots after the shot and whether J&J's vaccine should be available in the US. Last week, the CDC and the Food and Drug Administration paused distribution of J&J's shot following initial reports of six rare blood-clot cases, all in women under 60.

Advisors on the Friday call are reviewing data that suggests the risk of clotting is highest for women between the ages of 30 and 39. There have been no reports of these rare clots in men since the vaccine was authorized for use in the US.

A 25-year-old man had a clot after vaccination during J&J's clinical trials of the vaccine. So did a 24-year-old woman in the placebo arm of the trial (who did not receive the vaccine). Both recovered.

Health experts are eager to continue using the shot, the only one-dose vaccine authorized for use in the US. It's possible the vaccine will be unpaused with a warning to certain groups, such as younger women.

However, advisors say it's been difficult to make a clear call given the lack of data.

"It affects people that are younger in age, and seems to affect women more than men - so far," Dr. Michael Streiff, the medical director at the Johns Hopkins Anticoagulation Management Service and Hemostatic Disorder Stewardship Program, said during his presentation at the meeting.

A new theory about why certain people have a higher risk of clots

The chance of developing a clot for women under 50 was 7 in 1 million, while the risk was less than 1 in 1 million for women over 50 and for men, the data to be presented to the CDC panel suggested.

The panel of experts met last week to assess the situation but decided they wanted to see whether more of these unusual cases surfaced, as millions of Americans had only just gotten the J&J shot.

These cases of cerebral venous sinus thrombosis, or CVST, in women have popped up six to 13 days after immunization, meaning there was a good chance that people who were injected before the vaccine was paused last week could still develop clots.

It's still unclear what might be causing the clots, but a leading hypothesis is that these people are having a rare autoimmune reaction to the vaccine, similar to what happens to some people who take the blood thinner heparin.

Autoimmune heparin-induced thrombocytopenia is extraordinarily rare, Streiff said. "I couldn't give you a number. I can just tell you from my experience treating these patients, I've just never seen it," he said.

This type of clotting issue doesn't generally affect one gender more than another, Streiff added.

Advisors are keen to make the J&J shot work, but there are obstacles

Advisors have leaned toward reintroducing the vaccine with a warning label but wanted to get a sense of how rare or common these clots were before making a decision.

"Is this a needle in a haystack or the tip of an iceberg?" Dr. Sandra Fryhofer, a committee member, said during a meeting on April 16.

There are several widely used medications that carry a rare risk of blood clots, like contraceptive birth control. But the J&J situation is unusual because not only did people develop a rare form of blood clot in the brain, but they experienced low levels of the platelets that form blood clots.

Health officials have cautioned that a common treatment for busting blood clots, heparin, is dangerous in this situation.

Should the advisory committee decide on Friday to continue using J&J's vaccine, the shot could be back in wide circulation within days.