Moderna said an investigation by its Spanish CDMO partner Rovi had concluded that the foreign material discovered in the vials last week was probably splinters resulting from friction between two pieces of metal incorrectly installed in a production line.

Over several days, particles were also found in other vials in Japan’s Kanagawa, Okinawa and Gunma prefectures, but they were said to be from lots different from the original 1.63 million doses withdrawn last week for safety reasons. Beyond Rovi’s discovery, Japan's health ministry speculated that some of the incidents could have been due to needles being incorrectly inserted, breaking off fragments of the rubber stopper.

In a joint statement, Moderna, Rovi and Japanese distributor Takeda said corrective measures to the vaccine’s manufacturing process have been put into place. These include improved operating procedures for changeover of manufacturing lines. The companies noted, however, that grade 316 stainless steel "does not pose an undue risk to patient safety and does not adversely affect the benefit/risk profile of the product." 

Takahiro Kinoshita, vice-chair of the Covi-Navi vaccine information group, told the Reuters news agency that in his view it is unlikely that contamination of foreign substances led directly to the sudden deaths of two men three days after receiving doses from the batch. “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,” he added.

Japanese officials removed the affected vials from circulation. Besides the original find, more contaminants, of different material, have since been found in two other Moderna vaccine lots in Okinawa and Gunma prefectures. Commentators said they feared the incidents could reduce public confidence in Covid vaccines at a time when Japan is reporting some 25,000 new cases of the disease daily.

Author: Dede Williams, Freelance Journalist