Japan's Okinawa region suspended the use of Moderna's COVID-19 vaccine on Sunday after another contamination was spotted, the local government said.
It comes a day after the Japanese health ministry said it was investigating the death of two men who received shots from tainted Moderna batches—though the cause of their death is unknown.
The Okinawa prefecture, in southern Japan, said Sunday's vaccination programme was partially postponed.
"We are suspending the use of Moderna's COVID-19 vaccines as foreign substances were spotted in some of them," it said in a statement.
The lots affected by the contamination spotted in Okinawa on Saturday are different from the 1.63 doses suspended after the two deaths, according to local media reports.
That suspension came after the health ministry said two men, aged 30 and 38, died in early August after getting their second Moderna doses.
Those doses were drawn from one of three batches suspended by the government on Thursday after several vials were found to be contaminated.
The ministry said however that it was investigating the cause of death and it is unknown currently if there is a causal link with the vaccine.
"At this time, we do not have any evidence that these deaths are caused by the Moderna COVID-19 vaccine, and it is important to conduct a formal investigation to determine whether there is any connection," Moderna and its Japanese distributor Takeda said in a joint statement on Saturday.
The nature of the particles found in the vials, which were manufactured by a Moderna contractor in Europe, is also not known yet.
"The vials have been sent to a qualified lab for analysis and initial findings will be available early next week," Moderna and Takeda said.
The contractor, Spanish pharmaceutical firm ROVI, said in a statement Thursday that it was investigating the cause of the contamination in the batch, which was only distributed in Japan.
It added that the issue may have originated on one of its manufacturing lines.
Around 44 percent of Japan's population has been fully vaccinated, as the country battles a record surge of coronavirus cases driven by the more contagious Delta variant.
More than 15,800 people have died from COVID-19 in Japan, and large parts of the country are under strict virus restrictions.
