Marco Cavaleri, the head of the Office Biological Health Threats and Vaccine Strategy at the European Medicines Agency (EMA), confirmed that there is a "clear" link between vaccination with AstraZeneca and cases of thrombosis. The cause of this reaction, however, remains unknown.
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"We can now say that it is clear that there is a connection with the vaccine... We are trying to get an accurate picture of what is happening, to define in detail this syndrome due to the vaccine," he said.
"Among vaccinated people, there is a higher than expected number of cases of cerebral thrombosis with platelet deficiency among young people."
The researchers should also analyze the risk-benefit ratio for young women who often have thrombosis but suffer less from the effect of COVID-19.
"Let's not forget that even young women end up in intensive care for coronaviruses. Therefore, very meticulous work will be necessary to understand the risk-benefit ratio in favor of the vaccine for all ages."
The EMA will provide some preliminary definitions as early as this week but cannot yet indicate age limits for use of the AstraZeneca vaccine. These definitions will have to be provided by national authorities who have to manage vaccination campaigns even if they don't have all the data at their disposal.
"They may decide to reserve AstraZeneca for older people," Cavaleri said, adding that "they may apply a precautionary principle taking into account that they have other vaccines available."
The EMA official acknowledged that a European consensus on age limits would be preferable but will not be easy to achieve.
"I understand the European requirement to have a single strong voice, but I don't know if we will be able to have such an indication as early as this week. Surely the drug information will be updated, stating that some adverse events are related to the vaccine."