You are more likely to get a blood clot on birth-control pills than from the J&J vaccine - but not the same type of clot

• The CDC and FDA have recommended a pause in the rollout of J&J's COVID-19 vaccine.
• Of the 6.8 million Americans who got the shot, six women are known to have developed blood clots.
• Blood clots linked to birth-control pills are more common than that, but they're typically a different type.

The US paused its rollout of the single-dose Johnson & Johnson COVID-19 vaccine on Tuesday, following six reports of blood clots among people who got the shot.

All six cases involved women between 18 and 48 years old. They developed the clots six to 13 days after their shots, according to the Centers for Disease Control and Prevention and the Food and Drug Administration. So far, 6.8 million Americans have gotten the J&J vaccine - so if clots are indeed linked to the vaccine (which is not yet known) they're seen at a rate of less than one in 1 million.

To highlight how small that rate is, some experts have compared the statistic to rates of blood clots among women taking birth-control pills. As many as one in every 100 women taking birth control over a period of 10 years can experience a clot.

"As someone who got the J&J vaccine eight days ago, and who took oral contraceptives for 20 years, I'll take these odds," Angie Rasmussen, a virologist from Georgetown University, tweeted on Tuesday.

But it's challenging to directly compare the clots observed in people who got the J&J vaccine to those among women who take oral contraceptives, for two main reasons. The first is that in the six cases that US regulators are investigating, patients also showed low levels of blood platelets - cells that stop bleeding. That's not seen among women on birth-control pills who experience clots.

The second reason is that these are mostly not the same types of clots. The rare reaction that might be linked to the J&J shot is called cerebral venous sinus thrombosis (CVST), and it's a clot in the brain. The clots typically associated with birth-control pills, meanwhile, occur in veins inside the thigh or calf.

"This is a different clinical entity than blood clots associated with oral contraceptives," Dr. Melanie Swift, an internist and vaccine expert at the Mayo Clinic, told Insider.

The chart below compares the rate of known CVST clots among people who've gotten the J&J shot with the risk of that type of clot in the general population.

A second chart compares the rate of any type of blood clots - not only CVST - among COVID-19 patients, women on birth-control pills, and women in general.

Oral contraceptives raise the risk of clots, but they're still unusual

Estrogen, a hormone in oral contraceptives, is linked to as much as a four-fold higher risk of any type of blood clot. That's because it prompts the body to produce more of the plasma that helps blood stick together.

Still, pill-associated clots are quite unusual.

"For women taking combined oral contraceptives, blood clotting is a very small risk but a serious condition," Dr. Melanie Davies, a gynecologist in London and professor at University College London, told Insider. She said the risk can be compared to rare but serious events like a car crash.

"For 10,000 women over a year, one to five will have a blood clot anyway, and on the [pill] that rises to three to nine, so it is still less than one in 1,000 chance," she said. "It's also important to know that this is much less than the risk of getting blood clots in pregnancy and after childbirth."

As many as 65 out of every 10,000 new mothers experience a clot in the three months after childbirth.

A false comparison

Most birth-control-linked clots are found in women's legs, though they can also sometimes travel from the legs to the lungs.

"When you're looking at clots that are associated with birth control, those are usually going to be in the form of a deep vein thrombosis and very rarely a pulmonary embolism," Dr. Jessica Shepherd, an OB-GYN in Dallas who serves as the chief medical officer of VeryWell Health, told Insider.

Overall, your chance of developing deep vein thrombosis is one in 1,000 every year.

CVST, by contrast, occurs in the brain when the sinuses that drain blood from your head get blocked. About five people out of every million in the general population experience this each year. Women on birth control face a higher risk of CVST than men and than women who aren't on the pill.

Several studies have also shown that some COVID-19 patients get diagnosed with CVST.

For the FDA and CDC, concerns over blood clots among J&J recipients wasn't so much about the total number of cases, but rather that the patients also had low levels of blood platelets. According to Swift, the number of people who get this combination of symptoms is so small that it's "too low to provide a population estimate."

"This type of a combination of low platelets and blood clots has been very rarely seen in the past in other situations as an autoimmune phenomenon, but it's very, very rare," Dr. Peter Marks, director of the Center for Biologics Evaluation and Research at the FDA, said during a briefing on Tuesday.

'An abundance of caution'

Experts say people who've already gotten the J&J shot shouldn't panic.

"If they have received the shot and it has been over two weeks since getting the shot, they should not worry, as the problem seems to occur early," Dr. Paul Geopfert, a professor of medicine at the University of Alabama at Birmingham who worked on the J&J trial, told Insider. "It is also extremely rare, so it seems unlikely we will see many more in light of the safety pause."

Those who have gotten a J&J shot in the last two weeks, Geopfert added, can look out for clot-related symptoms: "CVST would be more associated with severe headaches, confusion, and loss of consciousness," he said.

Typical vaccine side effects like fatigue aren't likely to signal a clot.

The US health agencies said they recommended this pause "out of an abundance of caution" and to give healthcare professionals time to understand the potential risks and treat patients accordingly.

"I respect the independence of the FDA and their need to evaluate risk. But six out of 6.8 million is not a lot, and if they are going to land on 'we reviewed the data and everything is fine,' they need to be clear and quick and unequivocal," Hawaii Sen. Brian Schatz tweeted.

Dr. Catherine Schuster-Bruce contributed reporting to this story.