The substance reacts to magnets. Moderna confirmed receiving "several complaints of particulate matter" in vaccine vials in Japan but said it had found "no safety or efficacy issues" related to these reports. A Moderna spokesperson said the drugmaker believed a "manufacturing issue" at a plant in Spain was the cause.-GEG

Japanese health officials said the contaminated shots contained a "substance that reacts to magnets" which "could be metal."

From Nikkei Asia, "1.6m Moderna doses withdrawn in Japan over contamination":

About 1.6 million doses of Moderna's coronavirus vaccine have been taken out of use in Japan because of contamination reported in some vials, the Ministry of Health, Labor and Welfare said early Thursday.

Several vaccination centers have reported that vaccine vials contained foreign matter, according to an announcement from the ministry, which added it will seek to minimize the impact of the withdrawal on the country's inoculation program.

The ministry said later in the day that the substance that had been mixed in may have been metal. "It's a substance that reacts to magnets," a ministry official said. "It could be metal."