Japan's Takeda Pharmaceutical Co Ltd has said that "human error" caused metal contaminants to get into Moderna Inc Covid-19 vaccine doses, leading to a recall.

Takeda, which imports and distributes the vaccine in Japan, and Moderna said in a new report that a Spanish manufacturer discovered contaminants in some vials in July, but supplies from the same production were allowed to be shipped to Japan.

Japanese authorities in August suspended the use of three batches of Moderna shots containing 1.63 million doses after being notified of the contamination. Moderna conducted an investigation in partnership with Takeda and Spanish manufacturer Rovi, which operates the plant where the contamination occurred.

The new report said that the problem stemmed from "incorrect assembly and was due to human error specific to visually misjudging the required one millimetre gap between the star-wheel and the stopper" of machinery that put the tops on vaccine vials.

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