Why younger women have a higher risk of blood clots, whether or not they're vaccinated

• J&J's COVID-19 vaccine distribution is paused after reports of 6 rare, severe blood clots.
• The clots, seen with another blood condition, aren't the same as those linked to birth control.
• The birth control pill and pregnancy play into younger women's higher risk of blood clots.

The US rollout of Johnson & Johnson's COVID-19 vaccine is on pause as experts evaluate six cases of severe blood clots in the brain possibly linked to the shots.

It's unclear if the vaccines lead to these clots or if it's just a coincidence.

On one hand, the type of blood clot seen in conjunction with another blood condition in the six J&J cases is so rare in the general population its prevalence is unknown. That raises red flags that the shot could play a role.

On another hand, all six cases of post-J&J shot clots were in women between the ages of 18 and 48, a group at higher risk of blood clots anyway.

Similarly, most blood clot cases that occurred after AstraZeneca's COVID-19 vaccine in Europe were in women under 55. The European Medicines Agency has said the shots themselves weren't the cause.

Young women are at a higher risk of developing certain types of blood clots regardless of their vaccine status, Mary Cushman, a professor of medicine at the University of Vermont's Larner College of Medicine, told Insider. Hormonal birth control and pregnancy may play a role in this elevated risk.

Birth control pills and pregnancy both increase the risk of blood clots

Young women are at a relatively high risk for blood clots since they're most likely to take the birth control pill or get pregnant, which both increase the odds of developing a clot, Cushman told Insider.

Estrogen, a sex hormone that is in most oral contraceptives and spikes during pregnancy, is one reason for that elevated risk, Insider previously reported. The hormone has also been found to affect how immune cells respond to flu vaccines.

Specifically, the US Food and Drug Administration estimates that between 3 and 9 out of 10,000 women who take certain birth control pills will develop a blood clot each year, compared to 1 to 5 women per 10,000 who do not have risk factors for blood clots.

The risk of getting a blood clot as a side effect of the pill is higher than the likelihood of clotting due to the vaccine. Scientists have known about this risk factor for decades, however, it's still rare, as a recent TikTok video pointed out.

"Millions of women everywhere take the contraceptive pill, and amongst the hundreds of side effects that come with the contraceptive pill - one of which is death - there's a 6 in 10,000 chance of getting a blood clot," TikToker alysselizabeth said in the video.

Still, the risk of clotting post-vaccine versus the risk related to birth control isn't an apples-to-apples comparison. There's far more data on birth control-associated clots, but a lot of it is flawed and outdated. Plus, the specific type of brain clots seen after J&J vaccine, as well as its presentation with low platelet count, differentiates them from the clots most typical among birth control pill users.

"This is a different clinical entity than blood clots associated with oral contraceptives," Dr. Melanie Swift, co-chair of the Mayo Clinic COVID-19 Vaccine Allocation and Distribution Work Group, told Insider.

There are other risk factors for blood clots - including COVID-19

Obesity and genetic factors can also increase someone's risk of developing a blood clot, Cushman said. Physical activity and other healthy lifestyle choices can help manage that risk.

"It's always really important to stay active, avoid sedentary time, keep to a healthy weight, and have a healthy diet," Cushman said. "And if you're using an oral contraceptive, or if you have obesity, maybe just ratchet up your awareness a little bit, but don't avoid the shot."

She added that COVID-19 has been linked to deadly blood clots, too - so getting vaccinated will help prevent clots by lowering your chance of getting sick from the coronavirus.

"The bottom line is that the risk of adverse health consequences from COVID-19 far outweighs any risk of the shot," Cushman said.

Added Swift: "I would tell everyone to have confidence in the vaccine safety monitoring system in the US. What an amazing feat to have detected a signal of only 6 people among millions. ... We do not want this time out for one brand of vaccine to stall vaccination efforts overall because that would result in more needless lives lost to the pandemic."