Fact check: Comparison of blood clots from COVID-19 vaccines and birth control lacks nuance

Miriam Fauzia, USA TODAY
·8 min read

The claim: COVID-19 vaccines have a lower risk of blood clots than birth control

On April 13, amid reports of unusual blood clots following vaccination with Johnson & Johnson's COVID-19 shot, federal health officials called for a pause in its use pending further research.

That move was spurred by six women who developed cerebral venous sinus thrombosis, a very rare blood clot in the brain, and low blood platelets six to 13 days after vaccination. One 45-year-old Virginia resident died, and three women have been hospitalized, two of whom were in intensive care as of Thursday, according to The New York Times.

The news comes weeks after similar blood clot cases emerged in Europe with the AstraZeneca-Oxford vaccine, now totaling more than 200 and affecting mostly women under 60. On April 7, the European Medicines Agency concluded the disorder was a very rare side effect and recommended AstraZeneca's vaccine not be given to younger people.

While a few new cases have emerged since the J&J announcement, both the U.S. Food and Drug Administration and Centers for Disease Control and Prevention have said these blood clots are extremely rare – for J&J's vaccine, six confirmed reports out of 6.8 million doses administered as of April 12.

But these well-publicized clotting concerns have prompted experts and laypeople on social media to chime in, some of whom compared the vaccine clotting reports to the risk of blood clots from oral contraceptives in order to demonstrate the vaccine's safety.

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"Birth control has a far greater risk of blood clots than any Covid-19 vaccine so far!!!" claims one April 14 Instagram post.

That "far greater risk," according to one graphic shared on Facebook, is 500 to 1,200 cases among 1 million women, or 0.05% to 0.12% compared to AstraZeneca's purported 0.0004%. (The text accompanying the graphic claims J&J's vaccine has a "comparable" risk but acknowledges the percentages are not updated to reflect new cases.)

USA TODAY has reached out to the Instagram poster for further comment.

Taking birth control pills does carry a risk of blood clots, and they are indeed more common than clots observed after taking the AstraZeneca or J&J's vaccine. But this line of reasoning raises another question: Is the comparison justified?

Clotting is a complex system

Typically, blood clots form whenever there's an injury to a blood vessel. Tiny fragments of cells, called platelets, arrive on scene to plug up the hole. If you've ever had a scab on your skin, that's actually a dried blood clot.

But these biological Band-Aids can form even when there's no obvious damage. Estrogen — a reproductive hormone found in men (though not as much), women and hormonal birth control — enhances blood's ability to thicken. While it doesn't necessarily cause blood clots, estrogen can increase the risk by as much as three- to four-fold, according to the National Blood Clot Alliance.

Other factors that can affect one's chance of developing blood clots include genetic mutations in a clotting factor called V Leiden, which is associated with a three-fold risk increase; lifestyle choices such as smoking; preexisting conditions such as obesity and arteriosclerosis; pregnancy; and even sitting for too long.

For the affected vaccine recipients, it's still unclear what exactly triggered their blood clots, though many experts have pointed to similarities with a blood clotting disorder called heparin-induced thrombotic thrombocytopenia.

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Differences in clotting

The comparison between birth control and vaccine blood clots becomes problematic on closer inspection, experts say.

"It's comparing apples and cashews," said Dr. Jen Villavicencio, a family planning fellow at the American College of Obstetricians and Gynecologists, to The Lily.

"We're talking about a vaccine during a pandemic that's being administered to millions of people under an emergency use authorization, versus hormonal birth control, which has decades and decades and decades of science and safety data behind it, and very well-known risks associated with it that are known by all of the providers who would prescribe it."

Dr. Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security, agreed, telling Health while the comparison helps people contextualize risk, it does not address nuances like types of blood clots and treatment options.

Blood clots linked to birth control primarily affect the legs (called deep vein thrombosis), but in some rare instances, they can affect the lungs (pulmonary embolism). Vaccine-connected clots so far have predominantly occurred in the brain (some for the AstraZeneca vaccine occurred elsewhere).

As for treatment, a blood thinner such as heparin is used to stop a typical clot from growing and prevent any additional clots. Those linked to birth control can be treated with heparin, but it's not an option for vaccine-induced clots because it could actually worsen the condition.

There are also potential differences in terms of who is affected. While the six reported cases involved relatively young women (ages 48 and under), the occurrence of these rare blood clots could happen among men and older people, said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, during an April 13 media briefing. An exact patient profile is hard to predict and risk factors have yet to be determined.

As the post notes, however, the frequency of blood clots related to vaccine and birth control is also quite different.

The FDA estimates the risk of developing a clot for a woman on the pill is about 0.3% to 0.09%. By comparison, the clots associated with the J&J vaccine are six in 6.8 million, or about 0.00009%.

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Notably, patients are made aware of the clotting risks in birth control before taking it, Dr. Arthur Caplan, director of New York University Langone's Division of Medical Ethics, told MarketWatch.

Despite the possibility these cases are vaccine-related, it's worth noting COVID-19 infection itself can lead to abnormal blood clotting. One Oxford University study found infected patients had an eight to 10 times greater risk of cerebral venous sinus thrombosis compared to vaccinated individuals and 100 times greater than the general population.

In other words, people who might refuse the vaccine over blood clot concerns are actually facing an even higher risk of developing clots if they go on to contract COVID-19.

Our rating: Missing context

We rate the claim that blood clots linked to birth control are more common than those linked to vaccines as MISSING CONTEXT, because without additional information it could be misleading. based on our research. This statement is statistically accurate, as clots are more common in birth control. But experts warn it's not an ideal comparison given the differences in how the two applications came to be, how they are treated and where they appear.

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This article originally appeared on USA TODAY: Fact check: Birth control, vaccine blood clot comparison lacks nuance