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 on Friday, citing people familiar with the matter.
AstraZeneca, which had been buffeted by similar blood-clotting concerns for weeks, agreed, while Pfizer Inc and Moderna executive declined, saying their vaccines appeared safe, the report said.
The US federal health agencies had on Tuesday recommended pausing use of Johnson & Johnson's vaccine for at least a few days after six women under the age of 50 developed rare blood clots after receiving the shot.
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.
The specific adverse event has not been reported by those who received the Pfizer and Moderna vaccines, the companies had said.
J&J, AstraZeneca, Pfizer and Moderna were not immediately available for comments on the report.