Johnson & Johnson had reached out to rival Covid-19 vaccine makers to join in an effort to study the risks of blood clots, the Wall Street Journal reported, citing people familiar with the matter.

AstraZeneca which has been buffeted by similar blood clotting concerns for weeks, agreed, while Pfizer Inc and Moderna executives declined, saying their vaccines appeared safe, the report said.

Last week, European regulators said they were reviewing rare blood clots in four recipients of the J&J shot in the United States, after which, the WSJ report said, the company began to reach out to other vaccine makers.

Pfizer and Moderna also objected because they did not see the need to duplicate the efforts of agencies and companies that were already looking for blood clot cases and investigating the cause, according to the report.