"There's no evidence at this point indicating that the deaths are linked to the vaccines found to contain foreign particles," explained an official. "The deaths may have been accidental, but we decided to make the information public in order to secure transparency."
News of the fatalities came after three batches of Moderna were withdrawn from use as a precaution.
A pharmacist in Saitama Prefecture discovered black specks in Moderna vials while checking them before use in mid-August.
"They were much smaller than sesame seeds -- less than a millimeter. They looked like slices of something and were floating inside the bottle as I shook it," said Ishii Koki.
Similar foreign substances were subsequently found in 39 unopened Moderna vials at eight vaccination sites across Japan.
Health ministry officials say the particles are likely metal as they are attracted to a magnet, and probably entered the vials during the manufacturing process at a Spanish factory.
The ministry identified three batches of vaccines that may be tainted.Vaccines from those batches have been distributed to more than 880 vaccination sites across Japan. More than 500,000 have already been used.
"There's an extremely low risk of the vaccines with foreign matters causing serious health problems," said one ministry official.
Deaths of two men investigated
The Health Ministry revealed last week that two men -- aged 30 and 38 with no underlying health conditions -- both died three days after receiving vaccinations from one of the suspended Moderna batches.
Moderna and its distributer in Japan, Takeda Pharmaceutical, issued a joint statement on Saturday, saying “This is a tragic event, and the loss of life is something that we take very seriously. An investigation is being conducted with the greatest sense of urgency, and Takeda and Moderna will keep the public informed as we learn more."
Health Minister Tamura Norihisa told an NHK program Sunday that the government has also launched an investigation.
The ministry is calling on people who received vaccines with the stated lot numbers to seek medical attention if they experience any health abnormalities.
Data from the Prime Minister's Office shows that as of August 30, more than 55 percent of Japan’s population of 125 million has received at least one shot. More than 44 percent are fully vaccinated.