Scientists said on Friday that the overall risk of the particular clotting events that have drawn concern was extremely low: roughly one case in 600,000 recipients of the AstraZeneca vaccine in Britain. And it is difficult to know how common the cases are in the general population, given that it can be hard to diagnose. Scientists have said that case counts would inevitably rise among vaccinated people as doctors began looking more closely for the condition.
David Werring, a professor at University College London’s Institute of Neurology, said that the unusual presentation of the cases in vaccinated people was creating concern about possible links with the shot.
But, he said, “The key thing to remember is how rare these brain clots are, and how powerful the proven benefit of vaccination is against Covid.” He added that doctors and people who had received the AstraZeneca vaccine should be on the lookout for symptoms of the clotting events, like severe headaches or signs of a stroke.
“More research is urgently needed,” he said.
Melissa Eddy contributed reporting.