Two people died after receiving Moderna COVID-19 vaccine shots that were from the same production line suspended following the discovery of contaminants.
According to Japan's health ministry, two men in their 30s died recently within days of receiving their second Moderna doses. Each had a jab from one of three manufacturing lots suspended on Thursday.
However, the cause of the deaths is still being investigated.
Japan froze the use of 1.63 million Moderna doses delivered to 863 vaccination centres nationwide following reports of contaminants in some vials.
Spain's ROVI pharmaceutical firm, which is manufacturing Moderna’s COVID-19 vaccine, opened an investigation on Thursday to determine how vials of the vaccine sent to Japan were contaminated with foreign particles.
US vaccine maker Moderna is reportedly conducting a parallel investigation.
Japan's NHK broadcaster cited the ministry as saying that the contaminants were magnetic and appeared to be metallic particles. No safety issues linked to the three lots have been observed.