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Scientists and the US and the European drug regulators have been searching for an explanation for what is causing the rare but potentially deadly clots accompanied by low blood platelet counts, which have led some countries to halt or limit use of the AstraZeneca and J&J vaccines.

Other scientists have suggested competing theories for the clotting condition.

J&J, in an emailed statement said, "We are supporting continued research and analysis of this rare event as we work with medical experts and global health authorities. We look forward to reviewing and sharing data as it becomes available."

AstraZeneca declined to comment.

Researchers at the Goethe-University of Frankfurt and other sites explained in their paper that vaccines using a different technology known as messenger RNA (mRNA), such as those developed by BioNTech SE with partner Pfizer Inc and Moderna Inc, deliver the genetic material of the coronavirus spike protein only to fluid found inside cells, not to the nucleus of the cells.

“All mRNA-based vaccines should represent safe products,” the paper said.

The paper suggests that vaccine makers using adenovirus vectors could modify the sequence of the spike protein “to avoid unintended splice reactions and to increase the safety of these pharmaceutical products.”

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