A research group in Germany has presented a possible explanation for why the AstraZeneca and Johnson & Johnson COVID-19 vaccines sometimes trigger rare blood-clotting events. But not all experts are convinced the explanation is correct.

The group, led by Dr. Andreas Greinacher, head of the Institute of Immunology and Transfusion Medicine at University Hospital Greifswald, suggests a chain reaction that involves a preservative and certain proteins in the vaccines may be responsible for the rare blood clots.