AstraZeneca, Pfizer and rare blood clots

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Do COVID-19 vaccines increase the risk of a blood clot? HANNIBAL HANSCHKE/Getty Images
  • Two large studies have found a small increase in the absolute risk of rare types of blood clot 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 the age of 70.
  • The benefits of vaccination to protect against severe COVID-19 far outweigh the risks identified by researchers.
  • They found no evidence of increased risks after a first dose of the Pfizer-BioNTech COVID-19 vaccine.

In late February 2021, there were several reports 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 places, such as the veins in the head, and were often accompanied by low levels of platelets in the blood.

However, with the number of reported cases being so low, 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 increased.

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

Also, 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 UK have now found a small increase in the absolute risk of rare types of blood clot in the head. There was no evidence of an increased risk of more common types of blood clot.

“We were concerned that there were no more thromboses diagnosed in people after complications from the AstraZeneca vaccine were reported,” explained Dr. William Whiteley, Ph.D., of the Center for Clinical Brain Sciences at the University of Edinburgh in the UK, and lead author of one of the studies.

“That’s why we conducted the study using data prior to when thrombotic complications from the AstraZeneca vaccine were widely reported,” he said. Medical News Today.

“But our estimates [of the frequency of blood clots]and the ones done afterwards are similar, so maybe it’s not such a problem,” he added.

In the UK, a vaccination program using the Pfizer-BioNTech vaccine started on December 8, 2020, with the AstraZeneca vaccine added on January 4, 2021.

The program prioritized the clinically extremely vulnerable and those over 70, followed by people with chronic conditions, such as diabetes and high blood pressure, and those over 65.

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

Overall, 79% of participants were Caucasian, 51% female, and 84% under the age of 70.

The researchers compared the incidence of thrombosis before and after the first dose of vaccine. They then adjusted the figures 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. .

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

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

The corresponding figures for the Pfizer vaccine were 19% and 43% lower for venous thrombosis and 6% and 28% lower for arterial thrombosis.

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

In people under 70, rates of intracranial venous thrombosis – blood clots in a vein in the head – or hospitalization with low platelets were about twice as high within 28 days of a first dose of the drug. AstraZeneca vaccine.

However, as these events are extremely rare, the absolute increase in the number of events was very small.

Scientists estimate that after adjustments for other risk factors, the AstraZeneca vaccine could cause between 0.9 and 3 additional cases, depending on age and sex, for every million people vaccinated.

This small increase in risk is easily offset by the reduced risk of getting sick or dying from COVID-19 that the vaccine provides.

“Most people want accurate information about the pros and cons of the treatments they’re taking, and we’re giving them information,” Dr. Whiteley said.

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

After a first dose of the AstraZeneca vaccine, the study found no increased risk of intracranial thrombosis in people aged 70 or older.

After a first dose of the Pfizer vaccine, there was no increased risk for either the elderly or those under 70.

The results of the study are in OLP Medicine.

The authors concluded:

“For older populations, who are most vulnerable to COVID-19, we found no evidence of an increased risk of an event with ChAdOx1-S. In younger populations, which have lower morbidity and mortality from COVID-19, other available vaccines might be prioritized, particularly when the risk of COVID-19 is otherwise low.

The scientists plan to publish the results of their blood clot analyzes after second vaccine doses and after COVID-19 infections, in future articles.

In the second study, scientists led by the University of Edinburgh in the UK investigated the incidence 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 early reports of a possible link between the vaccine and CVST.

The background incidence of CVST is only 3 to 4 per million person-years in adults, but the risk of death for someone with CVST is about 4%.

Dr. Steven Kerr, Ph.D., senior data scientist at the University of Edinburgh, and colleagues linked electronic medical information from primary care and secondary care, as well as mortality and virology testing data.

The study encompassed 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 rate of CVST events for individuals for a period of 90 days before their vaccination with the 4 weeks after.

This is known as a self-controlled case serieswhere individuals act as their own control to account for other possible risk factors they might have that do not change over time.

There were 201 CVSTs in total. Of these, 81 CVSTs occurred during 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 initial very low incidence of CVST, which equates to one additional event for every 4 million people vaccinated.

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

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

The study appears in OLP Medicine.

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

“This has to be weighed against the risk associated with contracting COVID and the level and duration of protection that vaccines provide,” he said.

He added that the relative increased risk of CVST after a first dose of the AstraZeneca vaccine might be higher in younger people than in older people.

“It’s something that we may be able to study in the future, as we accumulate data that covers a longer period,” he added.

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

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