Japan has suspended use of 1.63 million doses of the Moderna shot.
The move comes more than a week after domestic distributor Takeda received reports of contaminants in some vials.
On Thursday (August 26) Japanese Prime Minister Yoshihide Suga sought to reassure citizens over use of the Moderna vaccines.
He said the country's inoculation rollout would be not be significantly hit:
"I have received a specific report from the health ministry about contaminants in Moderna's COVID-19 vaccine. I have instructed the health ministry to deal with the issue considering safety as a priority after analysing the current situation. Although I haven't got a detailed report as a conclusion, I received a report that it wouldn't have a significant impact on our inoculation plan."
Both Japan and Moderna said the suspension was just a precaution.
But the move prompted several Japanese companies to cancel worker vaccinations planned for Thursday.
Moderna said the contamination could be due to an issue on one of the production lines at its contract manufacturing site in Spain.
Japanese officials say some shots from the affected lot have been used, but didn't say how many individuals were affected.
The country is battling its worst wave of infections, driven by the Delta variant.
It has inoculated 54% of the population with at least one dose and fully vaccinated 43%, according to a Reuters vaccine tracker.