Regulators in the UK, Brazil, India and South Africa have allowed AstraZeneca to resume its clinical trials there.

The FDA, however, wants to determine whether similar side effects emerged in trials of other vaccines designed by AstraZeneca’s coronavirus vaccine partner, researchers at Oxford University, the sources said. That does not mean the agency believes there were safety issues associated with any of these vaccines, they added.

“It just shows that the FDA is being thorough,” said one of the sources.

Further complicating the situation is that the data requested by FDA is in a different format than what the U.S. regulator requires, two of the sources said.