AstraZeneca, Pfizer and rare blood clots
AstraZeneca, Pfizer and rare blood clots

AstraZeneca, Pfizer and rare blood clots

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Do COVID-19 vaccines increase the risk of blood clots? HANNIBAL HANSCHKE / Getty Images
  • Two large studies have found a small increase in the absolute risk of rare types of blood clots in the head after a first dose of the AstraZeneca COVID-19 vaccine.
  • An increased risk of a type called intracranial venous thrombosis only applies to people under 70 years of age.
  • The benefits of vaccination to protect against severe COVID-19 far outweigh the risks identified by the researchers.
  • They found no evidence of increased risks following an initial dose of the Pfizer-BioNTech COVID-19 vaccine.

In late February 2021, several reports of rare types of “thrombosis” – blood clots that block veins or arteries – appeared after the AstraZeneca (ChAdOx1-S) COVID-19 vaccine.

The blood clots were in unusual places, 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.

Researchers were also unsure whether the risk of common types of thrombosis also increased.

Part of the problem was that as COVID-19 vaccines became widely available, governments prioritized clinically vulnerable and elderly people who are already more prone to thrombosis.

In addition, mention of the risk of blood clots after vaccination may have made physicians more likely to diagnose thrombosis. This would give a false impression that they had become more common.

Two studies that collected data for millions of patients in the UK have now found a small increase in the absolute risk of rare types of blood clots in the head. There was no evidence of an increased risk of more common types of blood clots.

“We were concerned that more thrombosis could be diagnosed in humans after the complications of the AstraZeneca vaccine were reported,” he explained. Dr. William Whiteley, Ph.D., from the Center for Clinical Brain Sciences at the University of Edinburgh in the UK, and lead author on one of the studies.

“This was the reason why we did the study using data from before the date when the thrombotic complications of the AstraZeneca vaccine were widely reported,” he said. Medical news today.

“But our estimate [of the frequency of blood clots]and those made afterwards are similar, so it may not be such a concern, “he added.

In the UK, a vaccination program using 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 individuals over the age of 70, followed by people with chronic diseases, such as diabetes and high blood pressureand those over 65 years of age.

In the first study, researchers led by Dr. Whiteley the electronic health records for 46 million adults in the UK, of which 21 million received their first vaccine dose between December 2020 and March 2021.

Overall, 79% of the participants were white, 51% women and 84% under 70 years of age.

The researchers compared the incidence of thrombosis before and after the first dose of vaccine. They then adjusted the numbers to take into account other factors that may affect the incidence of blood clots, including age, gender, ethnicity, socioeconomic status, existing medical conditions, and medications.

Following adjustments, the overall risk of thrombosis was lower in the 28 days after the first dose of either the AstraZeneca or Pfizer-BioNTech vaccine compared to before vaccination.

For AstraZeneca, the risk of venous thrombosis was 3% and 42% lower in people under 70 and 70 years of age or older, respectively. The risk of thrombosis in arteries was 10% and 24% lower in these age groups, respectively.

The corresponding figures for the Pfizer vaccine were 19% and 43% lower for thrombosis in veins and 6% and 28% lower for thrombosis in arteries.

The researchers believe that the most likely explanation for these improvements is that vaccination significantly reduced the likelihood of COVID-19, which in itself can cause thrombosis, especially in the lungs.

In people under the age of 70, the incidence of intracranial venous thrombosis – blood clots in a vein in the head – or hospitalization with low platelet levels was approximately twice as high in the 28 days following 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 researchers estimate that after adjustments for other risk factors, the AstraZeneca vaccine can cause between 0.9 and 3 additional cases, depending on age and gender, for every million people vaccinated.

This small increase in risk is easily offset by the reduced risk of becoming ill or dying from COVID-19 provided by the vaccine.

“Most people want accurate information about the pros and cons of the treatments they take and we provide information to them,” said Dr. Whiteley.

“The overwhelming majority of people in the UK decide to be vaccinated against COVID-19 when offered a vaccine, and because of this, COVID-19 is less of a threat to all of us,” he added.

Following an initial dose of the AstraZeneca vaccine, the study found no increase in the risk of intracranial thrombosis in persons aged 70 years or older.

Following an initial dose of the Pfizer vaccine, there was no increased risk for either the elderly or younger than 70 years of age.

The results of the study are shown in PLOS Medicine.

The authors concluded:

“For older populations that are most vulnerable to COVID-19, we found no evidence of an increased risk of any incident with ChAdOx1-S. In younger populations that have a lower morbidity and mortality rate due to COVID-19, others may available vaccines become a priority, especially when the risk of COVID-19 is otherwise low. ”

The researchers plan to publish the results of their analyzes of blood clots after the second vaccine dose and after COVID-19 infections in future articles.

In the second study, researchers led by the University of Edinburgh in the UK investigated the occurrence of a rare type of blood clot in the brain called cerebral venous sinus thrombosis (CVST).

In 2021, several countries withdrew the AstraZeneca vaccine or restricted its use to the elderly following initial reports of a possible link between the vaccine and CVST.

The background incidence of CVST is only 3 to 4 per. million people in adults, but the chances of dying for a person with a CVST are about 4%.

Dr. Steven Kerr, Ph.D., a senior data researcher at the University of Edinburgh, and his colleagues linked electronic medical information from primary care and secondary care, plus mortality and virological test data.

The study included data from more than 11 million people in England, Scotland and Wales who received their first dose of a COVID-19 vaccine from December 2020 to June 2021.

The researchers compared the frequency of CVST events for individuals in a 90-day period before their vaccination with the 4 weeks after.

This is known as one self-directed case serieswhere individuals act as their own control to account for other possible risk factors that they may have that are unchanged over time.

There were 201 CVSTs in total. Of these, 81 CVSTs occurred in the follow-up period after the first dose of the AstraZeneca vaccine, or 16.34 events per million doses.

This represents a doubling of the very low initial incidence of CVST, which equates to one additional event for every 4 million vaccinated people.

There were 40 CVSTs among those who received a first dose of the Pfizer vaccine, or 12.6 events per day. million doses.

This suggests that there is no association between the Pfizer vaccine and CVST.

The study is shown in PLOS Medicine.

“It is important to understand that CVST is an extremely rare event that typically occurs only a handful of times per million people a year,” said Dr. Kerr MNT.

“This should be weighed against the risks associated with getting COVID and the level and duration of protection that vaccines offer,” he said.

He added that the relative increase in the risk of CVST after a first dose of the AstraZeneca vaccine may be higher in young people than in the elderly.

“This is something we may be able to study in the future as we gather data covering a longer period of time,” he added.

The authors say they will use the same method to investigate possible risks of CVST with the Moderna vaccine, which is now also in use in the UK, and the second and booster dose of all three vaccines.

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