The foreign substance detected in dozens of vials of Moderna Inc.’s COVID-19 vaccine in Japan is thought to have been tiny pieces of metal, but experts have dismissed concerns over significant health problems, saying that the chance of the foreign material actually entering the body is low.

The foreign substance was identified in 39 unused vials, all of which had the lot number 3004667, at eight workplace and large-scale inoculation sites in five prefectures — Aichi, Ibaraki, Gifu, Saitama and Tokyo — with the first discovery having come on Aug. 16. The government on Thursday decided to halt the use of vials belonging to that lot number — as well as those with the lot numbers 3004734 and 3004956, which were produced around the same time and at the same facility — affecting a total of 1.63 million shots.

Taro Kono, the minister in charge of the vaccine rollout, said Friday that more than half a million of the 1.63 million doses in question have already been administered.