Japan is investigating the death of two men who received jabs from batches of Moderna's Covid-19 vaccine suspended from use due to contamination, the health ministry said Saturday.

The men aged 30 and 38 died earlier this month after getting their second Moderna doses from one of three manufacturing lots suspended by the government on Thursday after several vials were found to be contaminated, the ministry said in a statement.

The ministry said the cause of death was still being investigated and "currently, causal relations with the vaccinations are unknown".

Both men contracted fever after receiving their vaccinations and neither had underlying health conditions or allergic history, the ministry said.

The suspension affects 1.63 million doses of Moderna Covid vaccines, which have been reportedly shipped to over 800 vaccination centres across Japan.

Takeda, which is in charge of sales and distribution of the Moderna shot in Japan, said it received reports from several vaccination centres that "foreign substances" have been found inside unopened vials.

Around 44 percent of Japan's population has been fully vaccinated, as the country battles a record surge of virus cases driven by the more contagious Delta variant.

Over 15,700 people have died from Covid-19 in the country, and large parts of Japan are under strict virus restrictions.