Japan is suspending the use of about 1.63 million doses of Moderna vaccine after contamination was found in some unused vials.
The health ministry said the contamination was reported at multiple vaccination sites.
Some doses may have been administered but no adverse health effects have been reported.
The doses were produced in Spain.
The Japanese distributor Takeda Pharmaceutical suspended the doses manufactured in the same production line as a precaution and asked Moderna to investigate the problem.
The suspension raises the prospect of supply problems in Japan, which is relying on three foreign-developed vaccines for its COVID-19 inoculation campaign.
The suspension also comes amid one of the country's largest surges in cases since the start of the pandemic. On Wednesday, the government expanded its coronavirus state of emergency for a second week in a row.
Four new prefectures added to a separate "quasi-emergency" status, meaning that 33 of Japan's 47 prefectures are now under some type of emergency measures.