Japan on Thursday halted use of 1.63 million Moderna COVID-19 shots after its health ministry received reports of foreign materials in the vials from several vaccination sites, Kyodo news agency reported.

The size of foreign particles confirmed in 39 vials is believed to be a few millimeters, with their elements being unknown, Kyodo said, adding that Moderna is investigating into it.

Japanese authorities said foreign substances have been confirmed since August 16 at eight vaccination sites across several prefectures.