SINGAPORE (BLOOMBERG, REUTERS, AGENCE FRANCE-PRESSE) - The AstraZeneca Covid-19 vaccine came under the spotlight following reports of rare blood clots, some of them fatal, among some adult vaccine recipients.
Concerns about the vaccine centred on an unusual type of blood clot in the brain called cerebral venous sinus thrombosis (CVST). There were also some cases of clots in the abdomen and in the arteries, which occurred together with low levels of blood platelets.
The European Medicines Agency said it had reports of 169 cases of the rare clotting disorder out of 34 million doses administered within Europe, roughly one for every 100,000 people under 60 years of age who were vaccinated. Most occurred in women under 55 and within two weeks of injection.
AstraZeneca said it was working with the British and European regulators to list possible brain blood clots as "an extremely rare potential side effect".
Among possible causes for the rare CVST clots being investigated are that the vaccine triggers an unusual antibody in rare cases, or a possible link with birth control pills.
But there is no definitive evidence.
Dr Andreas Greinacher, a scientist from Germany's University of Greifswald said his work indicates that neither birth control nor having a clotting factor mutation plays any role.
Some experts pointed out that younger women are more susceptible to such blood clots.
In the general population, the symptoms are about three times more common in women than men, and the median age of those afflicted is 33, the Financial Times reported.
However, the European Medicines Agency (EMA) executive director Emer Cooke said on Wednesday (April 7) the regulator were not able to pinpoint those at risk.
Dr Cooke said there was no available evidence of "specific risk factors such as age, gender or previous medical history of clotting disorders".
"A plausible explanation for these rare side effects is an immune response to the vaccine," she said.
Researchers at Oslo National Hospital had earlier suggested that cases might have been triggered by a "powerful immune response" to the vaccine.
An association of French scientists and doctors called "On the Side of Science" have said such an immune response could come from the accidental insertion of the needle into a vein in the upper arm, rather than muscle.
The EMA advised recipients to seek immediate medical assistance if they developed symptoms such as shortness of breath, chest pain, swelling in the legs, persistent abdominal pain, neurological symptoms, including severe and persistent headaches or blurred vision, and tiny blood spots under the skin beyond the site of injection.