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 and Johnson & Johnson.

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...