Why It's Tough to Directly Compare the J&J Vaccine Blood Clots to Those Linked to Birth Control

When side effects occur with medicinal products authorized for emergency use, like the COVID-19 vaccines, the government suspends the use of those products while it assesses the risks. This is exactly what happened in April 2021: For a period of 10 days, health officials from both the US Food & Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) recommended a temporary pause on the use of Johnson &Johnson's (J&J) COVID-19 vaccine over concerns about a potential link to a rare type of blood clot in six women who received the vaccine.

The pause was recommended "out of an abundance of caution" so that the CDC's Advisory Committee on Immunization Practices (ACIP) and the FDA could have a chance to further investigate these rare cases. The recommended standstill also served as an opportunity to make health care providers aware of the potential risk of this type of clot after a vaccine, as it requires a unique type of treatment.

The recommendation came as a shock to most—especially those included in the more than 6.8 million people who had already received their single-shot dose of J&J's vaccine at the time of the announcement. It also sparked many to flood Twitter with comparisons between the risk of blood clots linked to J&J's vaccine and other medications—oral contraceptives (aka, the pill), specifically.

It's true: Generally speaking, your risk of getting a blood clot from birth control is significantly higher than the potential risk of developing a blood clot from the J&J vaccine—but the types of blood clots we're talking about here aren't exactly the same. All blood clots aren't created equal, and there are actually several types out there, women's health expert Jennifer Wider, MD, told Health.

Here's what you need to know about your blood clot risk as it pertains to the J&J vaccine, compared to your risk of getting a blood clot from oral contraceptives—and why it's important to talk about these two with just a bit more nuance.

What Type of Blood Clot Has Been Linked to the J&J Vaccine—And What's the Risk?

The type of blood clot that caused the pause on the J&J vaccine is called a cerebral venous sinus thrombosis (CVST). A CVST is a rare type of blood clot that forms in the venous sinuses—spaces in the skull that let blood drain from the brain. It can lead to a very rare type of stroke that impacts less than five people in one million every year, per Johns Hopkins Medicine.

There were another nine documented cases of people who developed a CVST after getting the J&J vaccine in addition to the original six, according a FDA news release from April 2021. All 15 cases were women between the ages of 18 and 59, and each developed symptoms between six to 15 days after they were vaccinated. Those women also presented with thrombocytopenia—the term for a low blood platelet count. (Platelets are tiny blood cells that stop or prevent bleeding.)

On its website, the CDC reports that thrombosis with thrombocytopenia (TTS) occurs in about four out of one million cases. Given that this risk is very low, the FDA determined that the benefits of the Johnson & Johnson vaccine outweighed the risks and retracted its recommendation 10 days after issuing it.

OK, So What About Blood Clots and Birth Control Pills?

All medications come with risks, and oral contraceptives are no exception. According to data from the FDA, between three and nine women in every 10,000 who take oral birth control pills will develop a blood clot. (That's a 0.03 to 0.09 percent risk, if you prefer to think of it that way.) These chances are rather small.

The FDA also issued comparison data, showing that the likelihood of developing a blood clot when you're not on the pill is one to five in every 10,000 women. It's a little higher when you're pregnant—occurring in five to 20 in every 10,000 women—meaning that, in this context, pregnancy increases the risk of blood clots.

Here's the thing: The birth control data are talking about all blood clots here—not CVST in particular. The common blood clot linked to the pill is deep vein thrombosis (DVT). DVT is where a clot forms in a deep vein, usually in your leg or pelvis. And sometimes, in connection with oral contraceptives, pulmonary embolism (PE) can occur. A PE is when a clot breaks off, travels to your lungs, and causes a blockage in an artery there. "The mechanism [of DVT] is very different from the blood clot that was experienced after the Johnson & Johnson vaccine," said Dr. Wider, adding that the same goes for a PE.

Another big difference is in the way these blood clots are treated. With DVT, a drug called heparin (an anticoagulant or blood thinner) is often used to help prevent the clot from getting bigger, and to prevent additional clots from forming. However, regarding the blood clots and thrombocytopenia linked to the J&J vaccine, the FDA cautioned in a May 2022 fact sheet that "the use of heparin may be harmful and alternative treatments may be needed" in patients with suspected TTS.

That said, it is possible to develop a CVST while taking oral birth control pills. A 2015 meta-analysis published in the journal Frontiers in Neurology looked at 861 studies on CVST and found that the risk of developing the complication is 7.59 times higher in women who take oral birth control pills than in those who don't. Due to that, the study authors said that using oral contraceptive pills "increases the risk of developing CVST in women of reproductive age," but they also advise that more research is needed.

Should We Be Comparing These Blood Clot Risks?

It's difficult to directly compare the blood clots linked to the J&J vaccine and those linked to oral contraceptive use, but it can still be done in a responsible way—and may even be helpful.

"[The risks] need to be contextualized," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, told Health. "Yes, there are different types of blood clots, but this comparison helps the public understand that every medication has some risk," said Dr. Adalja, adding that these comparisons have value in helping people address the risks they find acceptable on a daily basis.

But sometimes, those quick, unexplained comparisons can go too far. "We can misuse this comparison," said Dr. Adalja, noting again that it's difficult to directly analyze the risk of blood clots linked to the J&J vaccine against those linked to birth control, since again, they are not the same type of clot nor do they have the same treatment options.

Dr. Wider agreed that the direct comparisons may not be entirely helpful, adding that there are also known risk factors for blood clots with birth control that aren't recognized as risk factors for the vaccine. "Women who smoke, have obesity, and lead a sedentary lifestyle are at increased risk of a blood clot when they are on birth control," Dr. Wider. "At this point, these are not underlying risk factors for the very rare clot that occurred post-Johnson & Johnson vaccine."

Overall, experts agreed that the pause was necessary in the distribution of the J&J vaccine because it allowed the FDA to protect public safety as scientists studied the relationship between the vaccine and the development of blood clots in the brain. With oral contraceptives, at least there is a known risk that people can weigh with their doctors before taking them, Dr. Wider pointed out.

Ultimately, your risk of getting a blood clot linked to the Johnson & Johnson vaccine is incredibly low based on CDC data. And so is your risk of developing a blood clot if you're taking the pill for birth control—but it is not quite that easy to compare these two types of clots.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.