Moderna has said that tainted batches of its Covid-19 vaccine sent to Japan were contaminated with stainless steel particles, but the company did not expect it posed “an undue risk to patient safety”.

In a joint statement with its Japanese partner Takeda, Moderna said the contamination in one of three suspended lots had been traced back to production line flaws at a factory run by its Spanish contractor, Rovid Pharma Industrial Services.

“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,” the statement said, according to AFP.

Metallic particles of this size injected into a muscle may cause a site reaction, but are unlikely to go beyond that, it added.