If borne out, the findings could lead to the jabs being altered in order to prevent the reaction.
The researchers, in a study which is yet to be reviewed by experts, said Covid-19 vaccines that employ adenovirus vectors - cold viruses used to deliver vaccine material - send some of their payload into the nucleus of cells, where some of the instructions for making coronavirus proteins could be misread, with the resulting proteins potentially triggering blood clot disorders in a small number of recipients.
Scientists and drug regulators have been searching for an explanation for the rare but potentially fatal clots accompanied by low blood platelet counts, which have led some countries to halt or limit the use of the AstraZeneca and J&J vaccines.
Johnson & Johnson, in a 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 that vaccines using a different technology known as messenger RNA (mRNA), such as those developed by BioNTech with Pfizer and Moderna, 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.
It suggests 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.”
It comes as doctors in the UK have been urged to look for signs of stroke in people receiving the Oxford/AstraZeneca vaccine after three young patients were admitted to hospital and one died.
Experts, including from the National Hospital for Neurology and Neurosurgery at University College London (UCL) Hospitals NHS Foundation Trust, told how two women in their 30s and a man in his 40s suffered clots in their large arteries, leading to ischaemic stroke, though they stressed the chances were small.
The team said the NHS must look out for patients with ischaemic stroke (where blood clots block arteries) within about one month of vaccination, saying they should be "urgently evaluated" for a very rare syndrome called vaccine-induced thrombosis and thrombocytopenia.
This condition needs to be rapidly diagnosed and managed by a team with a range of expertise, who have quick access to a range of drugs, they said. The experts stressed that the cases of stroke are very rare and that stroke is more common in people who catch Covid-19.
There have been 309 cases of major thrombosis with low platelet count suggesting VITT from more than 30 million doses of the AstraZeneca vaccine administered, they said.
The number of people who experience blood clots from VITT after a Covid-19 vaccine is therefore extremely low at about one per 100,000 doses.
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