COVID-19 vaccination in Japan has taken a hit after the country had to suspend the use of 1.63 million Moderna doses shipped to 863 vaccination centres. Japan's health ministry said it had launched a probe into the death of two men who had been administered doses from “contaminated” batches of the Moderna vaccine in August.

Moderna and its Japanese distributor Takeda Pharmaceutical Co said in a joint statement on Saturday said there was no evidence as of yet to conclude these deaths had been caused by the Moderna COVID-19 vaccine, and it was "important to conduct a formal investigation to determine whether there is any connection."

The first contamination was reported to Takeda on August 16, which led to the discovery of 39 vials containing foreign material, all from the same lot of 57,000 vials, or about 570,000 doses.

ALSO READ: After shortage of hospital beds and staff, US states run low on oxygen

On August 26, Japan said it had halted the use of lot 3004667 that included those vials and two other lots 3004734 and 3004956.

On Sunday, a vaccination centre in Okinawa prefecture reported that contaminants were found in a fourth lot, 3005293, according to the health ministry.

Black substances were spotted in syringes and a vial, while pink substances were found in a different syringe. The same day, a vaccine centre in Gunma prefecture reported a tiny, black substance in a Moderna vial from lot 3005236.

ALSO READ: Australia's Melbourne extends 6th lockdown amid Delta variant outbreak

The affected lots from Okinawa and Gunma have been temporarily suspended.

What Caused the Contamination?

The health ministry said on Monday that some of the contamination cases seem to have been caused by incorrect insertion of needles into vials.

There is no official word on the nature of the contaminants. However, Japan's public broadcaster NHK quoted health ministry sources as saying that the contaminants are believed to be metallic particles.

On August 27, Japan expanded its fourth coronavirus state of emergency till September 12 to include a total of 21 prefectures.