Post Misconstrues Public Health Awareness Campaigns About Blood Clots


SciCheck Digest

The Centers for Disease Control and Prevention and Pfizer tweeted in February about the dangers of blood clots in veins, which are relatively common and affect as many as 900,000 Americans each year. A story shared on social media, however, misleadingly linked those public health reminders to the COVID-19 vaccines. 

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Blood clotting in the deep veins, or deep vein thrombosis, is a serious and relatively common medical condition. In addition to damaging the valves of veins, which can cause pain and be debilitating, clots that form in the legs or pelvis can also travel to the lungs and block blood flow. That’s known as pulmonary embolism, which is often lethal. 

Collectively known as venous thromboembolism, DVT and/or PE affect up to 900,000 people in the U.S. each year and as many as 100,000 die, according to the CDC.

Online posts, however, have misleadingly zeroed in on public health messages about these blood clots from the CDC and the pharmaceutical company Pfizer to falsely suggest that the COVID-19 vaccines, including the two mRNA vaccines, are a major cause of clotting.

One COVID-19 vaccine authorized in the U.S. — the Johnson & Johnson vaccine — can cause a very particular blood clotting problem, but it is extremely rare. The condition has not been linked to either of the mRNA vaccines from Pfizer/BioNTech and Moderna, which account for the vast majority of doses administered in America. And in fact, evidence suggests COVID-19 vaccination prevents blood clots by protecting against COVID-19, which raises the risk of clotting.

The original article, which was shared online on Facebook, is from the Gateway Pundit, a conservative website that is a frequent source of misinformation online. The article discusses a Super Bowl-themed tweet the CDC sent out in advance of the big game reminding the public that DVT can happen to anyone, including athletes. The article then proceeds to misleadingly link the tweet to COVID-19 vaccination.

“Of course, no one has ever heard of this frequent phenomenon before the COVID vaccines,” the post reads. “The CDC failed to mention on their website the COVID-19 vaccines as one of the factors that can increase this risk.”

But there’s no reason for the CDC’s DVT page to include anything about the COVID-19 vaccines. The page, which was last updated in February 2020, before any COVID-19 vaccines existed and before the World Health Organization declared COVID-19 a pandemic, describes the main symptoms and risk factors for the clots.

It says, for example, that half of blood clots occur after hospitalization or surgery, and also highlights the risk of being immobile for long periods of time, such as when traveling or on bed rest. Some of these factors clearly can apply to athletes who get injured or who travel a lot, even if they otherwise are at lower risk because they are younger or more fit.

The Gateway Pundit article goes on to say that the Johnson & Johnson COVID-19 vaccine “increases the risk of developing a rare and deadly blood clotting condition in the brain.” The post then falsely contends that the mRNA vaccines have the same problem. (The issue has also been identified with the AstraZeneca vaccine, which, like the J&J vaccine, uses a viral vector design. But that vaccine has not been used in the U.S.)

“A study by Oxford University early last year also revealed that the number of people who developed blood clots after getting vaccinated was about the same for those who get Pfizer and Moderna vaccines as they are for the AstraZeneca, Market Watch reported,” the post said.

It’s true that MarketWatch reported that about an early draft of a study, but that’s a mischaracterization of the findings. All evidence suggests the mRNA vaccines do not cause clotting problems of any kind. Moreover, the rare clotting condition observed with the J&J and AstraZeneca vaccines, known as thrombosis with thrombocytopenia syndrome or vaccine-induced thrombotic thrombocytopenia, is different from the general concern about DVT in the population.


TTS or VITT involves blood clotting combined with low levels of blood platelets, typically after five days up to a month after vaccination with select vaccines. As of Feb. 24, the CDC and FDA had confirmed 57 cases of TTS following the J&J vaccine, after more than 18.4 million doses, or around 3 cases per million doses. Women between the ages of 30 and 49 are more likely to be affected, but the condition is still rare.

Dr. Adam C. Cuker, a hematologist with expertise in blood clots at Penn Medicine, called the post “ridiculous” and explained that while TTS/VITT can include blood clots in the legs or lungs, patients tend to present more frequently with clots in very unusual places, such as the cerebral sinuses in the brain or sometimes in the veins in the abdomen.

“In the general population, those are much, much less common places to develop clots than the legs and the lungs. But in this condition, it’s quite a bit more common to have clots in those unusual places,” he said.

Another key feature of TTS/VITT is that patients have antibodies to a protein called platelet factor 4. “That also is distinct from garden variety blood clots — DVT, PE — where patients don’t have those antibodies,” Cuker said. “So VITT is a very distinct entity that has a unique pathophysiology, a unique clinical presentation, a unique laboratory profile.” 

The treatment is also different. Usually, blood clots are treated with an anticoagulant such as heparin, but that drug can make TTS/VITT worse. Informing health care providers of this difference is one reason why U.S. regulators decided to pause administration of the J&J vaccine back in April after the CDC and FDA identified six cases of TTS in younger women, as we’ve written

And contrary to the notion that the CDC is not providing information about TTS/VITT, the agency has warnings about it in multiple places on its website. In mid-December, the CDC began recommending the Pfizer/BioNTech and Moderna vaccines over the J&J vaccine because of the low, but real, risk of the condition.

Importantly, no COVID-19 vaccine appears to cause blood clots generally, and the mRNA vaccines have not been associated with any kind of clotting problem.

“To date, CDC has detected no unusual or unexpected patterns of blood clots, such as deep venous thrombosis (DVT) and pulmonary embolism (PE) following immunization,” Dr. John Su, a medical officer for the CDC’s Vaccine Safety Team, told us in a statement. He did, however, say there had been 9 TTS/VITT deaths that were likely caused by the J&J vaccine.

“There are very large population studies involving millions of people that show that there is no increase in risk with the Pfizer and the Moderna vaccines in terms of thrombosis,” Cuker said. “So very, very clear evidence that those vaccines don’t cause clots.”

In fact, vaccination might prevent blood clots.

“There’s also very clear evidence that having COVID increases the risk of thrombosis,” Cuker said. So not only do the mRNA vaccines “not increase the risk of thrombosis, but there’s even a suggestion that they decrease it,” he added, citing an Israeli study published in the New England Journal of Medicine evaluating the risks of infection with the coronavirus and the safety of the Pfizer/BioNTech vaccine.

“It’s not because the vaccines themselves are anticoagulant, it’s because they prevent COVID,” Cuker said. “They prevent people from getting as sick with COVID, and that’s a major risk factor for clotting.”

Oxford Study Misinterpreted

As for the Oxford University study that purportedly found that “the number of people who developed blood clots after getting vaccinated was about the same for those who get Pfizer and Moderna vaccines as they are for the AstraZeneca,” the lead author of the paper told us that was “misleading and misinterpreted.”

The study, which was published in eClinicalMedicine in July, used electronic health records to look back in time and evaluate the risk of cerebral venous thrombosis and portal vein thrombosis following coronavirus infection, compared with mRNA vaccination or influenza infection. The main finding was that both clots were much more common after COVID-19 than with vaccination or after a bout of flu.

MarketWatch, the news site the Gateway Pundit cited, erroneously concluded from an early draft of the paper that blood clots were “as prevalent with Pfizer and Moderna vaccine as with AstraZeneca’s,” based on the study’s estimate of 4.1 cerebral venous thrombosis events per million people for the mRNA vaccines and the European Medicines Agency’s estimate of 5.0 events per million people for the AstraZeneca vaccine.

But that conclusion is faulty. The draft specifically said that “we cannot conclude that the mRNA vaccines studied here are associated with an increased risk of CVT” and that “far larger samples are needed to address this question.”

Maxime Taquet, the Oxford researcher who led the study, explained that his risk estimate for the mRNA vaccines was very uncertain and had broad confidence intervals — and therefore it does not mean that the mRNA vaccines show the same level of risk as the AstraZeneca vaccine.

“The broad confidence interval means that with our study we really cannot be sure what the true risk is among people receiving an mRNA vaccine and indeed it is perfectly compatible with the fact that the risk is not increased compared to the rest of the population,” he said. “The reason why the confidence interval (i.e. our degree of uncertainty) is so big is precisely because this is such a rare event and we need even larger populations to clearly estimate the risk.”

Because of the confusion, Taquet said his group decided to remove the point estimates in their published paper.

The published paper, which was available well before the Gateway Pundit article was written, also makes this point, noting that “the results of this study are consistent with the hypothesis that these vaccines are not associated with an increased rate of CVT.”

Finally, the Gateway Pundit article attempts to tie in a Feb. 14 tweet from Pfizer, which also warned the public about DVT. The article implies Pfizer’s tweet might somehow be related to the CDC’s tweet and COVID-19 vaccination. But there is no evidence that Pfizer’s message had anything to do with the agency’s tweet — and as we’ve already established, there is nothing linking the Pfizer/BioNTech vaccine to blood clots.

Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.


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Pfizer Inc (@pfizer). “Deep vein thrombosis (#DVT), a blood clot in a deep vein, can travel to the lungs, leading to a pulmonary embolism (#PE). Symptoms of PE include difficulty breathing and chest pain. Contact your doctor if experiencing symptoms—this is no time to wait.” 14 Feb 2022.

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