AstraZeneca Plc’s COVID-19 vaccine looks like it’s headed for an additional global trial as the drugmaker tries to clear up uncertainty and confusion surrounding favorable results in its current study.

The company wants the new test to confirm the 90% efficacy rate that the shot showed in a portion of an existing trial, Chief Executive Officer Pascal Soriot said. It’s favoring that option rather than adding an arm to a separate study that’s already underway in the U.S.

Questions are mounting over one of the fastest-moving shots after the company acknowledged that a lower dosage level that appeared more effective resulted from a manufacturing discrepancy. The company and its partner, the University of Oxford, didn’t initially disclose the error and other key details, leading to concern over their transparency.