German scientists now believe they have identified the cause of rare blood clots in some recipients of the Oxford/AstraZeneca and Johnson & Johnson vaccines, the i newspaper reports.
Those vaccines have been linked to rare, but serious, blood clots over the last few months.
In the UK, the AstraZeneca vaccine has been linked to 309 clotting cases and 56 deaths out of 33 million shots.
The research was led by Professor Rolf Marschalek, of Goethe University in Frankfurt, who was contacted by Johnson & Johnson, asking for guidance on how to fix the issue.
The researchers, in a study not yet reviewed by experts, said the two vaccines employ adenovirus vectors, or cold viruses, to deliver vaccine material.
Some of their payload enters the nucleus of cells, where some of the instructions for making coronavirus proteins can be misread.
The resulting proteins could potentially trigger blood clot disorders in a small number of recipients, they suggest.
The scientists say they hope their research can be used as a basis to fix the problem.
The process is different with mRNA vaccines, such as those made by Pfizer and Moderna, because the genetic material of the spike protein is sent directly to the cell fluid and does not enter the nucleus.
US and 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 Johnson & Johnson vaccines.
Other scientists have suggested different theories for the clotting condition.
A Johnson & Johnson spokesman 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