Japan has suspended the use of about 1.63 million doses of the Moderna vaccine due to contamination.
The health ministry said "foreign materials" were found in some doses of a batch of roughly 560,000 vials.
Takeda Pharmaceutical, which sells and distributes the vaccine in Japan, said Moderna had put three batches on hold "out of an abundance of caution".
It said an issue at a manufacturing contract site in Spain was the likely cause, but did not elaborate.
"To date, no safety or efficacy issues have been identified," Moderna said, adding that it would work with regulators and Takeda to investigate the matter further.
There are no details of what the "foreign objects" are, but Takeda described it as particulate matter, after which it said conducted an emergency examination.
Reports of contamination also came from seven other vaccination centres, according to the Japan Times newspaper, with 39 vials - or 390 doses - found to have been affected.
The health ministry has published the batch numbers so that people who had received their shot before the suspension could check if they have got a potentially contaminated shot, Japan Times added.
Japan is battling a spike in Covid cases, with eight more prefectures placed under a state of emergency on Wednesday.
Its capital, Tokyo, is currently hosting the Paralympic Games.
The country has already approved the Pfizer and AstraZeneca vaccines for use - it only started using Moderna in May.
Just over 40% of Japanese people are fully vaccinated and around 50% have received one dose.