After 1.6 million doses of the Moderna (MRNA) vaccine were suspended due to a foreign substance in the vials last week, Moderna is recalling three batches of the shot because of contamination.

Moderna plans to recall the vaccine doses in conjunction with its Japanese partner Takeda Pharmaceutical, which oversees the sale and distribution of the shot in Japan. The two companies said they are currently conducting an investigation at the Spanish factory where the COVID vaccine batches were produced.

Moderna and Takeda believe that the contamination occurred during the production process of putting stoppers on the vials of the vaccine. The companies said that the probable cause was most likely from friction from two pieces of metal in the machinery that puts the stoppers on the vials.

The foreign substance in the vials was confirmed by the companies to be stainless steel.

"Stainless steel is routinely used in heart valves, joint replacements and metal sutures and staples. As such, it is not expected that injection of the particles identified in these lots in Japan would result in increased medical risk," Takeda and Moderna said in a joint statement.

As many as 180,000 possibly contaminated doses were administered before the vaccine suspension was initiated, Kyoda News reported last month. But pharmaceutical and health ministry officials say they do not believe the high-grade stainless-steel contamination found in the Moderna doses poses a health risk.

Following the contamination issues, two men died after receiving the Moderna shot. Japanese health officials do not believe there is a link between the vaccine contamination and their deaths, though the cause of death for the two men is still being investigated.

To date, Japan has reported over 1.5 million coronavirus cases and over 16,000 COVID-19 deaths, according to data from Johns Hopkins University.

As of Thursday's premarket hours, shares of Moderna were trading at $394.47, up $4.53, or 1.16%.

The BioNTech/Pfizer and Moderna vaccines
The BioNTech/Pfizer and Moderna vaccines. AFP / Daniel ROLAND