Study led by FDA researchers didn’t find that the Pfizer-BioNTech COVID-19 vaccine caused blood clots, contrary to viral claims on social media
Certain COVID-19 vaccines, specifically the viral vector vaccines like the AstraZeneca and J & J vaccines, are associated with an increased risk of a blood clotting disorder called vaccine-induced thrombotic thrombocytopenia, which can be fatal. At the moment, there isn’t evidence indicating that mRNA COVID-19 vaccines are associated with the same risk. However, it’s important to also consider that getting COVID-19 itself increases a person’s risk of developing blood clotting problems to a greater extent than the viral vector COVID-19 vaccines.
Misleading: Although the study observed an association between the vaccine and certain blood clotting disorders when comparing vaccinated people with historical pre-COVID data, this association disappeared after the researchers controlled for potential confounding factors like healthcare-seeking behavior and COVID-19 itself. This suggests that the initial association wasn’t a true association with the vaccine.
An article published by the Epoch Times on 17 December 2022 claimed that the U.S. Food and Drug Administration (FDA) discovered the Pfizer-BioNTech COVID-19 vaccine is “linked to blood clots”. This claim is based on a study led by researchers at the FDA and published in the journal Vaccine on 1 December 2022. The Epoch Times article received more than 6,400 engagements on Facebook, including more than 3,200 shares, according to social media analytics tool CrowdTangle.
It’s not the first time that The Epoch Times published misinformation about COVID-19 vaccines, as previous reviews by Health Feedback attest to. As we will explain below, the article left out the information necessary to accurately interpret the study’s findings, effectively misrepresenting its conclusions.
The study included more than 30 million individuals aged 65 years and older, who had received a COVID-19 vaccine since December 2020. This population accounted for the administration of more than 17 million of Pfizer-BioNTech COVID-19 vaccine doses, as well as more than 16 million doses of the Moderna vaccine and more than 630,000 doses of the Janssen vaccine (also called the J & J vaccine).
This population was monitored for 14 adverse events, including acute myocardial infarction (AMI; also known as a heart attack), and a variety of blood clotting disorders, namely deep vein thrombosis (DVT), pulmonary embolism (PE), disseminated intravascular coagulation (DIC), and immune thrombocytopenia (ITP).
To determine whether the population vaccinated against COVID-19 had a greater risk of experiencing these 14 adverse events, and whether different vaccine brands were associated with different levels of risk, the authors compared the COVID-vaccinated group with other populations. One was a historical control group, representing the baseline risks in the general population from the pre-COVID period (2017 to 2019) within the same age group (65 years and above).
Another was a control group that comprised individuals vaccinated against the flu, “to assess differential healthcare utilization”. This is because people who tend to seek out health-related interventions, like vaccination, can differ in important ways from people who don’t, and such differences may go on to influence an individual’s risk of developing disease independent of the healthcare intervention. For example, a person’s frequent use of healthcare services may reflect their socioeconomic status or their ability to access healthcare.
Another group comprised data gathered from 1 January 2020 to 10 December 2020, defined by the authors as the peri-COVID period.
The researchers reported no association between the Moderna and Janssen COVID-19 vaccines and any of the 14 adverse events when compared with the historical control group.
But they observed that the Pfizer-BioNTech COVID-19 vaccine was associated with a higher risk of AMI, PE, DIC, and ITP when compared to the historical control group derived from the pre-COVID period. Interestingly, no association with the blood clotting disorder DVT was observed.
These observations form the basis for the Epoch Times article’s claim that the Pfizer vaccine is linked to blood clots.
However, once the Pfizer-vaccinated group was compared to either the flu-vaccinated group or when monthly variations in the rates of such adverse events were accounted for, the associations for AMI, DIC, and ITP disappeared.
And when the peri-COVID period was compared with the Pfizer-vaccinated group, the associations for PE and DIC disappeared. The peri-COVID period is relevant since getting COVID-19 itself carries a higher risk of blood clotting disorders than COVID-19 vaccination[2,3]. As such, the post-COVID period may be marked by an overall increased incidence of such illnesses compared to the pre-COVID period.
These findings suggest that the initially detected associations, obtained by comparisons with historical pre-COVID data, were the result of confounding factors—such as differences in healthcare-seeking behavior and monthly variations in the baseline rates of such adverse events—rather than true associations with the Pfizer vaccine. Confounding factors are variables that affect the outcome of an experiment, but aren’t the variables being studied in the experiment.
Yazan Abou-Ismail, a hematologist at the University of Utah Health who specializes in blood clotting disorders, also pointed out in a Twitter thread that the claim is inaccurate and the study “cannot be used to claim causality”.
Here’s the link to the original paper: https://t.co/WkDom6GNPG
The authors used Medicare/Medicaid data to analyze 14 different adverse outcomes after 34 million vaccine doses in recipients age >65. The controls used for comparison were HISTORIC controls, pre-COVID & peri-COVID.
— Yazan Abou-Ismail, MD – د. يزن أبواسماعيل (@DoctorYazanA) December 18, 2022
As the authors acknowledged in the Discussion section of their study, their findings should be interpreted cautiously and don’t prove that vaccination caused the observed adverse events. Another factor that the study didn’t account for was the underlying comorbidities in the groups studied, which can influence the risk of disease as well.
Instead of explaining the implications of the study’s limitations, the Epoch Times article quoted cardiologist Peter McCullough, who previously spread COVID-19 misinformation, as saying that the study “corroborates the concerns of doctors that the large uptick in blood clots, progression of atherosclerotic heart disease, and blood disorders is independently associated with COVID-19 vaccination.” As our review demonstrates above, a complete reading of the study would show that this is far from the case.
In summary, the study by Wong et al. didn’t show that the Pfizer-BioNTech COVID-19 vaccine was responsible for causing blood clots in vaccinated people. Although the study observed an association between the vaccine and certain blood clotting disorders when comparing vaccinated people with historical pre-COVID data, this association disappeared after the researchers controlled for potential confounding factors like healthcare-seeking behavior and COVID-19 itself.
- 1 – Wong et al. (2022) Surveillance of COVID-19 vaccine safety among elderly persons aged 65 years and older. Vaccine.
- 2 – Hippisley-Cox et al. (2021) Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study. BMJ.
- 3 – Taquet et al. (2021) Cerebral venous thrombosis and portal vein thrombosis: A retrospective cohort study of 537,913 COVID-19 cases. eClinicalMedicine.