The Moderna coronavirus vaccine programme in Japan has been hit by a series of contamination incidents, prompting it to recall 1.63m doses found to contain metal fragments.
Other potential contaminants have been identified in separate batches over the past week but so far no injuries as a result of those vaccines have been reported.
The problems come as Japan battles its fifth and worst wave of infections. On Tuesday, new cases topped 25,000 and seriously ill patients climbed above 2,000, both record highs.
The discovery of contaminating materials led to the withdrawal on 26 August of 1.63m doses of the vaccine from three batches manufactured in Spain.
More than 500,000 doses from the faulty batches had already been administered, said Taro Kono, the minister in charge of the vaccine rollout, last Friday.
On Wednesday, Moderna and local distributor Takeda Pharmaceutical issued a joint statement on the contamination, saying: “The rare presence of stainless steel particles in the Moderna Covid-19 vaccine does not pose an undue risk to patient safety and it does not adversely affect the benefit/risk profile of the product.”
Moderna is investigating how the particles entered the vial at the manufacturing plant operated by its contractor Rovi Pharma.
The companies insisted that even if the metal fragments were injected into people, they would cause nothing more than a localised reaction. Health experts in Japan have suggested that if the fragments are visible to the naked eye they are too large to pass through a needle.
There have been other cases of potential contamination. On 28 August, use of the Moderna vaccine was temporarily suspended on the island of Okinawa after black and pink substances were found in different vials from a batch separate to the previously recalled ones.
Health minister Norihisa Tamura later said that the substances were likely pieces of rubber stoppers on the vials that had entered the liquid due to incorrectly inserted needles.
Then on 29 August, health workers spotted small specks of a black substance in vials at a vaccination centre in Gunma, north of Tokyo. Vaccinations resumed from those batches, which were different to those previously identified as contaminated.
On 2 September, black particles were found in Moderna vials in Kanagawa, south of Tokyo, by a pharmacist administering doses there. The substance has yet to be identified.
No issues from Moderna doses injected in Okinawa, Gunma or Kanagwa have been reported so far.
At the same time, an investigation into the deaths of two men in their 30s with no known health issues who died days after receiving their second dose from the contaminated batches in mid-August is ongoing. So far, no link with the vaccines has been established.
More than 12.2m Moderna doses have been administered in Japan, where the majority of vaccine centres have been using the Pfizer jab.
The AstraZeneca vaccine, which has been produced locally, was only recently approved due to concerns about reports of blood clots in other countries. Japan delivered millions of the domestically manufactured doses to Asian countries as part of an international programme.
Japan was due to take delivery of 50m doses it had secured from Moderna by the end of the month. It is not clear how much of an impact the contamination issues will have on that plan.
More than 130m vaccine doses have been administered in Japan and 46% of the population is fully vaccinated, with 57% having received at least one shot.