AstraZeneca Plc’s COVID-19 vaccine trial remains on hold in the U.S. as regulators examine a serious side effect suffered by a U.K. patient, Reuters reported on Monday.

The drugmaker and its partner, the University of Oxford, restarted the U.K. trial of the vaccine on Saturday after it was halted on Sept. 6. The U.K. Medicines Health Regulatory Authority recommended the study resume after an independent review of the safety data had triggered the pause.

However, enrollment of new study participants and other trial procedures remain stopped in the U.S. pending an investigation by the Food and Drug Administration and an independent safety panel, Reuters reported citing unnamed sources.

AstraZeneca and the FDA didn’t immediately respond to requests for comment.

The interruption of the closely watched study raised concerns about the viability of one of the fastest-moving experimental shots seeking to provide protection from the pandemic. The race to develop a vaccine has compressed what is normally a decade-long process into a matter of months.

U.S.-traded shares of AstraZeneca were up 0.5 per cent at 3:39 p.m. in New York.

The drugmaker is one of several companies taking part in the U.S. government’s Operation Warp Speed program to fast-track a coronavirus vaccine. Its shot has been considered likely to be one of the earliest to reach the market.