• A review of vaccine safety data found blood clots were most common in women under 55.
  • Getting a COVID-19 vaccine will not increase your risk of blood clots.
  • But the birth control pill, pregnancy, and obesity can lead to a higher preexisting risk.
  • See more stories on Insider’s business page.

After a preliminary review of AstraZeneca COVID-19 vaccine data, the European Medicines Agency (EMA) concluded last week that the shot was not associated with an increased risk of blood clots.

The number of clots reported after vaccination was actually lower than expected in the general population, the EMA said in a press briefing Thursday.

But one subset of the vaccinated population experienced more blood clots than normal: women under 55 years old.

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.

Sabine Straus, head of the EMA’s safety committee, said in Thursday’s briefing that an increased risk of clotting due to birth control pills is “always a distinct possibility,” and the EMA may investigate it further in the near future.

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

Young women are 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.

In fact, the risk of getting a blood clot as a side effect of the pill is much higher than the likelihood of clotting due to the vaccine. Scientists have known about this risk factor for decades, and it recently came up in a TikTok video.

“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. 

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.

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

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.

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