finally over rare, but sometimes fatal, blood clots reported in some recipients.
Those concerns led several European countries to first restrict the use of AstraZeneca in older age groups, then suspend it over reports of blood clots, only to roll it out again last month after the European Medicines Agency issued a preliminary opinion that the benefits of the vaccine outweighed the risks.
As doctors reported a higher incidence of serious blood clots in younger people, some countries decided to stop administering the shot to anyone younger than 55.
Europe’s concerns over the vaccine’s side effects are also likely to threaten global inoculation efforts, with much of the developing world depending on the AstraZeneca vaccine to tackle the pandemic. The shot is the cornerstone of Covax, a program designed to make vaccine access more equitable worldwide.
The vaccine appeared to be causing an immune reaction in which antibodies bind to platelets, activating them, German doctors and the European Medicines Agency have said. Those platelets, in turn, were causing the formation of dangerous clots in certain parts of the body, including in veins that drain blood from the brain, leading in some cases to a rare type of stroke.
Why the antibodies develop in these people is not known, doctors have said. Some component of the vaccine, or excessive immune reaction — or both — could be the cause, they said.
No pre-existing conditions are known to make patients more vulnerable to this clotting disorder after a vaccination, European regulators said.