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- Two large studies have found a small increase in the absolute risk of rare types of blood clot in the head following a first dose of the AstraZeneca COVID-19 vaccine.
- An increased risk of a type called intracranial venous thrombosis only applied to individuals under 70 years of age.
- The benefits of vaccination to protect against severe COVID-19 far outweigh the risks that the researchers identified.
- They found no evidence of increased risks following a first dose of the Pfizer-BioNTech COVID-19 vaccine.
In late February 2021, several reports emerged of rare types of “thromboses” — blood clots that block veins or arteries — after the AstraZeneca (ChAdOx1-S) COVID-19 vaccine.
The clots were in unusual locations, such as the veins in the head, and often accompanied by low platelet levels in the blood.
However, because the number of reported cases was so small, it has been difficult to estimate the increased risk for populations as a whole.
Scientists were also unsure whether the risk of common types of thrombosis also increases.
Part of the problem was that when COVID-19 vaccines became widely available, governments prioritized clinically vulnerable and older people, who are already more prone to thromboses.
In addition, publicity about the risk of blood clots after vaccination may have made doctors more likely to diagnose thromboses. This would give a false impression that they had become more common.
Two studies that pooled data for millions of patients in the United Kingdom have now found a small increase in the absolute risk of rare types of blood clot in the head. There was no evidence of any increased risk of more common types of blood clot.
“We were worried that there might be more thromboses diagnosed in people after the complications of the AstraZeneca vaccine had been reported,” explained Dr. William Whiteley, Ph.D., of the Centre for Clinical Brain Sciences at the University of Edinburgh in the U.K., and the lead author of one of the studies.
“This was the reason we did the study using data from before the date the thrombotic complications of the AstraZeneca vaccine were widely reported,” he told Medical News Today.
“But our estimates [of the frequency of blood clots], and those done afterward are similar, so this may not be such a concern,” he added.
In the U.K., a vaccination program that employed the Pfizer-BioNTech vaccine began on December 8, 2020, with the AstraZeneca vaccine added on January 4, 2021.
The program prioritized extremely clinically vulnerable individuals and those over 70 years, followed by people with chronic illnesses, such as diabetes and high blood pressure, and those over 65 years.
In the first study, researchers led by Dr. Whiteley analyzed the electronic health records of 46 million adults in England, 21 million of whom had their first vaccine dose between December 2020 and March 2021.
Overall, 79% of the participants were white, 51% female, and 84% under 70 years.
The researchers compared the incidence of thromboses before and after the first dose of vaccine. They then adjusted the figures to account for other factors that might affect the incidence of blood clots, including age, sex, ethnicity, socioeconomic status, existing medical conditions, and medications.
After adjustments, the overall risk of thromboses was lower in the 28 days after the first dose of either the AstraZeneca or Pfizer-BioNTech vaccine, compared with before vaccination.
For AstraZeneca, the risk of thromboses in veins was 3% and 42% lower in those under 70 and those 70 years or older, respectively. The risk of thromboses in arteries was 10% and 24% lower, respectively, in these age groups.
The corresponding figures for the Pfizer vaccine were 19% and 43% lower for thromboses in veins, and 6% and 28% lower for thromboses in arteries.
The researchers believe that the most likely explanation for these improvements is that vaccination substantially reduced the likelihood of COVID-19, which can itself cause thromboses, especially in the lungs.
In people under 70 years of age, the rates of intracranial venous thromboses — blood clots in a vein in the head — or hospitalization with low platelet levels were roughly twice as high in the 28 days after a first dose of the AstraZeneca vaccine.
However, because these events are extremely rare, the absolute increase in the number of events was very small.
The scientists estimate that after adjustments for other risk factors, the AstraZeneca vaccine may cause between 0.9 and 3 extra cases, depending on age and sex, for every million people vaccinated.
This small increase in risk is easily offset by the reduced risk of becoming sick or dying from COVID-19 that the vaccine provides.
“Most people want accurate information about the pros and cons of the treatments that they take, and we are providing information for them,” said Dr. Whiteley.
“The overwhelming majority of people in the U.K. decide to be vaccinated against COVID-19 when offered a vaccine, and because of this COVID-19 is less of a threat to us all,” he added.
After a first dose of the AstraZeneca vaccine, the study found no increase in the risk of intracranial thromboses among people 70 years or older.
Following a first dose of the Pfizer vaccine, there was no increased risk either for people older or younger than 70.
The results of the study appear in PLOS Medicine.
The authors concluded:
“For older populations, who are most vulnerable to COVID-19, we found no evidence of increased risk of any event with ChAdOx1-S. In younger populations, who have a lower morbidity and mortality due to COVID-19, other available vaccines might be prioritized, especially when the risk of COVID-19 is otherwise low.”
The scientists plan to publish results of their analyses of blood clots after second vaccine doses, and after COVID-19 infections, in future papers.