Big data study refutes claim about Covid vaccines posing high risk of blood clots

"The study reinforces the safety and importance of staying current with Covid-19 vaccinations."
Sejal Sharma
Covid-19 has claimed the lives of 69 million people
Covid-19 has claimed the lives of 69 million people

lakshmiprasad S/iStock 

One of the most common fears around the Covid-19 vaccine has been thrombotic thrombocytopenia, a rare disorder that causes blood clots to form in blood vessels in the body. The fear wasn’t unsubstantiated since many cases of blood clots or pulmonary embolisms emerged after people were inoculated with adenoviral vector-based vaccines. Later, vaccine-induced thrombotic thrombocytopenia (VITT) was recognized as a rare complication of specifically adenoviral vector-based vaccines.

The Centers for Disease Control and Prevention also later issued a preference for mRNA vaccines. 

This has, in turn, bolstered anti-vaxxers. Now, anti-vaxxers have been around for as long as vaccines. The term ‘anti-vaxxers’ refers to people who refuse to get vaccinated due to perceived fears and various other reasons. Basically, they want to call their own 'shots', if you will.

But now, much to the chagrin of anti-vaxxers, a new study published in the Journal of Clinical and Translational Science claims that covid vaccines pose minimal risk of blood clots.

Minimal risk of blood clots from the virus

As per World Health Organization’s estimates, Covid-19 has claimed the lives of 6,908,554 people worldwide. A total of 13.3 billion vaccine doses have been administered so far.

It’s a well-established fact at this point that immunization safeguards people from harmful diseases and conditions. But often, there are side effects to certain vaccines as well. 

This recent study, undertaken by researchers at the University at Buffalo in New York, was launched to investigate whether receiving a Covid vaccine put people at a higher risk for developing blood clots.

The sample size of the study was 855,686 veterans aged 45 years and above who had been administered at least one dosage of the Covid vaccine. The control group included 321,676 people, who also were veterans but unvaccinated. None of the participants had a positive Covid test.

The results of the study indicated that there is only a trivial increased risk of blood clots with the current Covid-19 vaccinations used in the United States in veterans older than age 45. The risk is significantly less than Venous thromboembolism (VTE) (or blood clots) risk amongst hospitalized covid patients or with other triggering risk factors for VTE. 

The study adds that the net clinical benefit favors vaccination and that it should be encouraged. “The excess risk was about 1.4 cases per million patients vaccinated. Given the fact that the rate of VTE with COVID-19 is several orders of magnitude greater than the trivial risk from vaccination, our study reinforces the safety and importance of staying current with COVID-19 vaccinations,” said Peter L. Elkin, MD, first author of the paper in a press release.

Study Abstract:


It is important for SARS-CoV-2 vaccine providers, vaccine recipients, and those not yet vaccinated to be well informed about vaccine side effects. We sought to estimate the risk of post-vaccination venous thromboembolism (VTE) to meet this need.


We conducted a retrospective cohort study to quantify excess VTE risk associated with SARS-CoV-2 vaccination in US veterans age 45 and older using data from the Department of Veterans Affairs (VA) National Surveillance Tool. The vaccinated cohort received at least one dose of a SARS-CoV-2 vaccine at least 60 days prior to 3/06/22 (N = 855,686). The control group was those not vaccinated (N = 321,676). All patients were COVID-19 tested at least once before vaccination with a negative test. The main outcome was VTE documented by ICD10-CM codes.


Vaccinated persons had a VTE rate of 1.3755 (CI: 1.3752–1.3758) per thousand, which was 0.1 percent over the baseline rate of 1.3741 (CI: 1.3738–1.3744) per thousand in the unvaccinated patients, or 1.4 excess cases per 1,000,000. All vaccine types showed a minimal increased rate of VTE (rate of VTE per 1000 was 1.3761 (CI: 1.3754–1.3768) for Janssen; 1.3757 (CI: 1.3754–1.3761) for Pfizer, and for Moderna, the rate was 1.3757 (CI: 1.3748–1.3877)). The tiny differences in rates comparing either Janssen or Pfizer vaccine to Moderna were statistically significant (p < 0.001). Adjusting for age, sex, BMI, 2-year Elixhauser score, and race, the vaccinated group had a minimally higher relative risk of VTE as compared to controls (1.0009927 CI: 1.007673–1.0012181; p < 0.001).


The results provide reassurance that there is only a trivial increased risk of VTE with the current US SARS-CoV-2 vaccines used in veterans older than age 45. This risk is significantly less than VTE risk among hospitalized COVID-19 patients. The risk-benefit ratio favors vaccination, given the VTE rate, mortality, and morbidity associated with COVID-19 infection.

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