While the study found no increased risk of blood disorders with the Pfizer-Biontech vaccine, it did find that AstraZeneca was associated with a small increase in the risk of developing a blood disorder.
It’s called immune thrombocytopenic purpura, or ITP, which can cause not only bruising in some cases but also dangerous bleeding in others. The risk was estimated at 1.13 cases per 100,000 people receiving their first dose, up to 27 days after vaccination.
The researchers said that the condition is treatable, and none of the cases in the vaccine recipients were fatal. They stressed that the benefits of the vaccine far outweigh the small risks, and noted that Covid itself is much more likely than the vaccine to cause ITP, according to the New York Times.But the researchers also wrote that although the risks for AstraZeneca were small, “alternative vaccines for individuals at low risk of COVID-19 may be justified when the supply permits.”
It was not surprising, the researchers said, noting that there were small increases in risk with measles, mumps and rubella vaccinations, and hepatitis B and influenza injections.
In a commentary published with the study, experts in blood disorders said ITP may be difficult to diagnose and that the potential connection needs further analysis. But they wrote, “However, the risk of ITP vaccination at the suggested rate appears to be significantly lower than the many risks associated with COVID-19 itself.”
The researchers said they could not rule out a link to stroke, but there were not enough cases to analyze.
In turn, Professor Aziz Sheikh, senior author of the study, from the University of Edinburgh, said during a press briefing that brain clots are “as rare as a chicken’s teeth.”
AstraZeneca and Johnson & Johnson use so-called viral vectors to transfer genetic material into recipient cells, and some researchers have suggested that vectors may lead to rare blood disorders. Whether there is an unknown connection.
The authors of the Scotland study said they did not know whether their findings on AstraZeneca had any effects on the Johnson & Johnson vaccine, which they did not study.