With two Covid-19 vaccines now under scrutiny for possible links to very rare cases of blood clots in the brain, US government scientists are focusing on whether the specific technology behind the shots may be contributing to the risk, according to Reuters.

In Europe, health regulators said last week there was a possible link between the AstraZeneca Plc vaccine and 169 cases of a rare brain blood clot known as cerebral venous sinus thrombosis (CVST), accompanied by a low blood platelet count, out of 34 million shots administered in the European Economic Area.

The US Food and Drug Administration recommended temporarily halting use of the Johnson & Johnson vaccine after reports of six cases of CVST in women under age 50 among some 7 million people who received the shot in the United States.

Scientists are working to find the potential mechanism that would explain the blood clots. A leading hypothesis appears to be that the vaccines are triggering a rare immune response that could be related to these viral vectors, FDA officials said.