Japan has suspended the use of more than 1.6 million doses of Moderna COVID-19 vaccine made in Spain over contamination fears.
The Japanese health ministry reported on Thursday that "anomalous substances" were found in unused vials from multiple vaccination sites.
Some doses might have been administered, but no adverse health effects have been reported, officials said.
But Takeda Pharmaceutical, a Japanese drugmaker in charge of sales and distribution of the vaccine, said it decided to suspend the use of 1.63 million doses manufactured in the same production line as a safety precaution.
The company said it had asked Moderna to conduct an emergency investigation and told medical institutions in Japan not to administer vaccines from the Moderna-affiliated plant in Spain.
The Japanese authorities have also released the reference numbers of the batches in question so that patients who have received these vaccines can consult medical specialists if they experience any discomfort.
The decision has raised concerns of a supply shortage as the country tries to accelerate vaccinations amid a COVID-19 surge.
Daily new coronavirus cases are hitting new highs in many parts of the country and severely straining the health care system.
Chief Cabinet Secretary Katsunobu Kato told reporters the government and Takeda are discussing ways to minimise the impact on Japan’s vaccination progress.
"We will do the utmost in order to avoid any impact on vaccination progress, especially at worksites and large-scale centers," Kato said.
Japan relies entirely on foreign-developed vaccines produced by Moderna, as well as Pfizer and AstraZeneca.
About 43% of the Japanese population have been fully vaccinated, while more than 54% have received a single dose.