TOKYO — Moderna Inc. and its Japanese partner are recalling more than 1 million doses of the U.S. drug maker’s coronavirus vaccine after confirming that contamination reported last week was tiny particles of stainless steel.

Takeda Pharmaceutical Co. is in charge of sale and distribution in Japan of the Moderna vaccine. The two companies said an investigation at a Spanish factory that produced the vials in question concluded the contamination occurred in the process of putting stops on the vials.

The companies on Aug. 26 announced suspension of 1.63 million doses produced at the line after reports of contamination. Japanese officials said about a half million people had received shots from the Moderna vials before the problem surfaced.

The trouble comes at a time Japan is pushing to accelerate vaccinations amid rising infections that are straining the Japanese health care system.

Pharmaceutical and health ministry officials say they do not believe the high-grade stainless steel poses health risks.