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The increased risk of being admitted to hospital or of dying from blood clots for a person with coronavirus is “much higher” than in people who received a first dose of vaccine, a study found.
The researchers said their work is the first to compare the AstraZeneca and Pfizer vaccines with risks of Covid-19 on such a large scale.
The document, which is the largest study of its kind on the issue, comes after a coroner concluded that an award-winning BBC radio presenter died of complications from AstraZeneca vaccination.
Lisa Shaw died in May, just over three weeks after receiving her first dose.
The coroner said she had developed vaccine-induced thrombosis and thrombocytopenia, which was described as “a rare and aggressive complication associated with the AstraZeneca vaccine, which was the underlying cause of her death.”
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The new research looked at more than 29 million people aged 16 or older who received a first dose of either vaccine in England between December and April.
Their results suggest that the risk of thrombocytopenia – a condition in which the patient has a low number of cells, called platelets, that help blood to clot – in a person with coronavirus is almost nine times higher than in a person who received a dose of AstraZeneca shot.
Professor Aziz Sheikh, one of the study participants, said the increased risk of thrombocytopenia seen in their work is similar to that of other vaccines commonly used in the UK, such as the flu shot.
They estimated that out of 10 million people vaccinated with AstraZeneca jab, there would be 107 more cases of thrombocytopenia within 28 days of vaccination, compared to 934 in people infected with the virus.
Scientists found an association between people vaccinated with the Pfizer vaccine and an increased risk of stroke, but the risk was more than 10 times higher in people infected with the virus.
There have been around 143 additional cases of ischemic stroke per 10 million people with Pfizer, compared with 1,699 cases among those with Covid-19.
The article, published in the BMJ, said that for every 10 million people who received a stroke from AstraZeneca, there were about seven more cases of CVST, while there were 20 in people with Covid- 19.
For blood clotting in a vein (venous thromboembolism), they estimated around 66 excess events per 10 million people vaccinated with AstraZeneca, compared to around 12,614 excess events in people with Covid.
There was no association with blood clots in an artery (arterial thromboembolism) for either vaccine, but there were some 5,000 excess events per 10 million people infected with Covid, they said. .
Researchers found no association between AstraZeneca and the risk of stroke, or Pfizer with an increased risk of thrombocytopenia.
Professor Julia Hippisley-Cox, professor of clinical epidemiology and general medicine at the University of Oxford, said the increased risks detected were only for a short period after vaccinations, compared to a longer period in case of infection with the virus.
She said: “For strokes, with Pfizer, it wasn’t until 15 to 21 days after vaccination that there was an increased risk.
“And for the thrombocytopenia with AstraZeneca, it was eight to 14 days. They were therefore very specific, short-lived, while associations with infection generally seemed to be over a period of 28 days after infection ”.
Professor Sheikh, director of the Usher Institute at the University of Edinburgh, said the study is important for “contextualizing” the safety results “against the risks if you get the infection” he has. said other studies had not been done to date.
Dr Martina Patone, statistician at the University of Oxford, said: “We looked at hospitalizations or deaths from blood clots within 28 days of receiving either vaccine, and what we have found is an increased risk with both vaccines, but also that the risk of blood clots is much higher if you have caught Covid-19, before or after the vaccination ”.
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