After enduring months of public criticism over its COVID-19 vaccine, AstraZeneca PLC finally caught a moment of reprieve this week when U.S. trial data showed the shot to be highly effective — working well in older people, and raising no safety concerns.

The good news lasted less than a day.

A group of experts meant to work hand in hand with the drugmaker on its trial safety contacted U.S. government agencies late Monday to express concern. The results made public just hours earlier may be outdated, they said, in a stunning departure from protocol. Astra promised up-to-date numbers in the next 48 hours.