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A healthcare worker administers a dose of the Moderna Inc. Covid-19 vaccine at a mass vaccination site in Saitama City, Saitama, Japan, on Monday, Aug. 23, 2021. Japan's Prime Minister Yoshihide Suga said last week that he expects 60% of the population to be fully vaccinated by the end of next month and plans to distribute enough doses to inoculate 80% of eligible people by early October. Photographer: Noriko Hayashi/Bloomberg Image Credit: Bloomberg

Tokyo: Japan’s health ministry said on Wednesday that contaminants found in suspended Moderna Inc COVID-19 vaccines were particles of stainless steel, and it did not expect they would pose an additional health risk.

Japan suspended the use of 1.63 million doses of the Moderna vaccine last week after being notified of contamination that local media reported was metallic. The findings on Wednesday were the result of an investigation by domestic distributor Takeda Pharmaceutical Co, the ministry said in a statement.

“Stainless steel is routinely used in heart valves, joint replacements and metal sutures and staples. As such, it is not expected that injection of the particles identified in these lots in Japan would result in increased medical risk,” Takeda and Moderna said in a joint statement.

The use of three lots of Moderna vaccines was suspended on Thursday after 39 vials were found to contain foreign material.

All the vials came from a single lot, but two others were suspended as a precaution as they all came from the same manufacturing line of Spanish pharmaceutical company Rovi , which bottles Moderna vaccines.

The most probable cause of the contamination was related to friction between two pieces of metal in the machinery that puts stoppers on the vials, Takeda said, citing a joint investigation with Moderna and Rovi. The material was confirmed to be grade 316 stainless steel.

The three suspended lots were the only ones affected by the manufacturing issue and Rovi has taken corrective measures, the statement said.

Use of other doses of the Moderna vaccine from different batches also was temporarily halted in three regions in Japan this week. In some cases, foreign substances have been found in unused vials, whereas others appear to be caused when bits of the vials’ rubber stopper break off when needles are incorrectly inserted.

The contamination issue gained more attention after the health ministry said on Saturday that two men, aged 38 and 30, died in August within days of receiving their second Moderna doses. Each had received a dose from one of the suspended lots.

The cause of death in the two cases is still being investigated. Takeda said there was no evidence the fatalities were caused by the vaccine.

“The relationship is currently considered to be coincidental,” the company said in the statement.