“Although there are some risks, clearly the risks of these events are very rare,” said Aziz Sheikh, a co-author of the study and a professor of primary care research at the University of Edinburgh in Scotland. “And the biggest point is that the risks associated with Covid-19 are orders of magnitude higher, really.”
The study examined the electronic health records of people given a first dose of a coronavirus vaccine during the first five months of England’s inoculation campaign. Of those 29 million people, nearly 1.8 million people also tested positive for the coronavirus before or after being vaccinated. The study compared the risk of blood clots shortly after vaccination to the risk during other periods, as well as to the weeks after someone developed Covid-19.
After a first shot of the AstraZeneca vaccine, people were at slightly increased risk of certain blood clots, as well as a condition characterized by a low level of platelets that can leave them prone to abnormal bleeding. A first shot of the Pfizer vaccine appeared to put people at slightly elevated risk of strokes caused by blockages within a blood vessel.
And both vaccines were linked, though at low numbers, to the very rare clots that prevent blood from draining out of the brain.
Even so, those risks were far smaller than those linked to developing Covid-19. For example, the authors said, for every 10 million people given a first dose of the AstraZeneca vaccine, about 66 more people than normal would develop clots starting in a vein. But among the same number infected with the virus itself, 12,614 more people than normal would develop those clots.