Japanese health ministry officials say one of the foreign substances found in batches of Moderna coronavirus vaccine vials proved to be stainless steel.
Moderna contacted the ministry on Wednesday to explain stainless steel particles formed as a result of friction between metal in the production line, which was caused by an inappropriate set-up.
Unopened vaccine vials from three lots accounting for 1.62 million doses have been suspended from use after foreign matter was found in some of the vials sent to the over 900 vaccination sites across Japan since mid-August.
Takeda Pharmaceutical, the distributor of the Moderna vaccine in Japan, decided to recall the doses starting Thursday after holding talks with ministry officials.
At the same time, Moderna and Takeda maintain there is only a small likelihood of increased medical risk as the metal is used in joint replacements and pacemakers.
Another foreign substance found in the Moderna vials in Tokyo, Okinawa and elsewhere was identified as fragments of rubber stoppers that likely broke off from the vials at the production line and when the vials were opened.
Moderna released the results of its investigation of that incident and said the fragments pose no safety risks.