Two young men in Japan died after receiving Moderna Covid-19 vaccine shots that were stopped from use after a week of having found to be contaminated, according to Japanese news broadcaster NHK .

The men in their 30s died this month within days of receiving their second Moderna doses, Japan’s health ministry said in a press release.

Each of these persons had a shot from one of three manufacturing lots suspended on Thursday. Both the persons had fever the day after their second dose and died two days later. The causes of death are being investigated.

The contaminants found in some vials in Japan are believed to be metallic particles, public broadcaster NHK reported, citing health ministry sources.

Japan put on hold the use of 1.63 million doses of US pharma giant Moderna’s COVID-19 vaccine on Thursday following reports of contaminants in some vials.

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Takeda, the Japanese company that distributes the vaccine in the country, detected the contaminants on August 16, but informed the government after a week on Wednesday which means several more people must have got the shots from the contaminated lot. 

Takeda informed the government that the 7-day delay occurred as it needed time to gather information on which vials were affected and where they were in the country.

However, Fumie Sakamoto, the infection control manager at St. Luke's International Hospital in Tokyo, cautioned against drawing a connection between the shots and the fatalities reported on Saturday.

"There may only be a temporal (time) relationship between vaccination and death," Sakamoto told Reuters. "There are so many things we still don't know to make any conclusions on these two cases."

"At this time, we do not have any evidence that these deaths are caused by the Moderna COVID-19 vaccine," Moderna and Takeda said in a statement on Saturday. "It is important to conduct a formal investigation to determine whether there is any connection."