German researchers on Wednesday said that based on laboratory research, they believed they have found the cause of the rare but serious blood clotting events among some people who received COVID-19 vaccines made by AstraZeneca Plc (AZN.L) and Johnson & Johnson (JNJ.N). The researchers, in a study not yet 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 can be misread. The resulting proteins could potentially trigger blood clot disorders in a small number of recipients, they suggest. Scientists and U.S. 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 J&J vaccines. read more Other scientists have suggested competing theories for the clotting condition. Johnson & Johnson, 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 (PFE.N) and Moderna Inc (MRNA.O), 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.”