TOKYO: Moderna Inc's COVID-19 vaccine contamination woes in Japan have widened with another million doses being suspended, after foreign substances were found in more batches and two people died following shots from affected lots.

The suspension of Moderna supplies, more than 2.6 million does in total, comes as Japan battles its worst wave of COVID-19 yet, driven by the contagious Delta variant, with new daily infections exceeding 25,000 this month for the first time amid a slow vaccine rollout.

The latest reports of vaccine contamination came from the Gunma prefecture near Tokyo and the southern prefecture of Okinawa, prompting the suspension of two more lots in addition to the 1.63 million doses already pulled last week.

A tiny black substance was found in a Moderna vaccine vial in Gunma, an official from the prefecture said, while in Okinawa, black substances were spotted in syringes and a vial, and pink material was found in a different syringe.

The fresh suspensions followed a government report on Saturday that two people died after receiving Moderna shots that were among lots later suspended.

The government had said that no safety or efficacy issues had been identified and that the suspension was a precaution. The causes of death are being investigated.

"It is unlikely, in my opinion, that contamination of foreign substances led directly to sudden deaths," said Takahiro Kinoshita, a physician and vice chair of Cov-Navi, a vaccine information group.

"If the contaminated substances were dangerous enough to cause death for some people, probably many more people would have suffered from some symptoms after the vaccination.

"However, further investigations are definitely needed to evaluate the harm of the particular doses in question."

'LOOK AT THE BIGGER PICTURE'

Japan earlier halted the use of 1.63 million Moderna doses, shipped to 863 vaccination centres nationwide after the domestic distributor, Takeda Pharmaceutical, received reports of contaminants in some vials.

Some 500,000 people received shots from those supplies, Taro Kono, the minister in charge of the vaccine push, has said.

Moderna and Spanish pharma company Rovi, which bottles Moderna vaccines for markets other than the United States, said at the time that the contamination could be due to a manufacturing issue in one of Rovi's production lines.

Takeda said on Monday that the investigation is ongoing.

The affected vaccines in Gunma are from a Moderna lot that is different from those whose use has already been suspended, the Gunma official said.

Vaccines from the same lot have been administered to 4,575 people in Gunma, but the prefecture has heard no reports of ill health, the official said.

The contamination "is a serious problem" and there is need to investigate, but given rising COVID-19 cases, Moderna vaccinations should "continue with appropriate precautions", said Nicholas Rennick, an Australian doctor practicing at the NTT Medical Centre in Tokyo.

Severe COVID-19 cases are at record levels in Japan, leaving many people to recuperate at home amid a shortage of critical care beds. Only 44% of its population has been fully vaccinated, lagging vaccination rates of several developed countries.

Japan is looking into the possibility of mixing shots of AstraZeneca's vaccine with those developed by other drugmakers to speed up vaccination.

"We've got thousands of Delta variant patients around Japan as we speak, spreading the virus, and so many of the population remain unvaccinated and unprotected," Rennick said.

"We have to look at the bigger picture."