A total of 1.63 million doses of the Moderna vaccine were suspended from use by Japanese officials after concerns of possible contamination.

As many as 39 reports of a foreign substance that appeared to be floating in the vaccine vials were reported by medical workers at eight vaccination sites in Ibaraki, Saitama, Tokyo, Gifu, and Aichi, Japan, according to Kyoda News.

The substance was described as tiny particles that were several millimeters in size, but it was unknown at the time of writing what the particles were made of, Business Insider said.

Moderna told Reuters that it “confirms having been notified of cases of particulate matter being seen in drug product vials of its COVID-19 vaccine. The company is investigating the reports and remains committed to working expeditiously with its partner, Takeda (Pharmaceutical), and regulators to address this.”

As many as 180,000 possibly contaminated doses were already administered before the suspension of the vaccine was initiated, Kyoda News reported. No adverse effects from the inoculations were reported, NPR said.

The vaccine doses fell under the lot numbers 3004667, 3004734, and 3004956 and were produced from three vaccine batches produced in Spain, according to the Japan Times.

Japanese Health officials said they halted the use of the 1.63 million COVID vaccine doses temporarily as a safety precaution and have requested that Moderna conduct an emergency investigation, NPR reported.

Moderna told Reuters that the contamination could be the result of a manufacturing issue at one of the production lines at its Spanish contract manufacturing site.

The company also told the news outlet that the suspended batches were only meant for distribution in Japan.

Japan is administering the Moderna, Pfizer, and AstraZeneca COVID vaccines.

A total of 43% of Japan’s 126 million citizens have been fully vaccinated against the virus, according to the Reuters vaccine tracker. All vaccinations were stopped in the country on Thursday as a result of the suspended Moderna doses, Reuters said.

The country is currently averaging over 25,000 COVID-19 infections a day, with 21 prefectures under a state of emergency as of Wednesday until Sept. 12, up from 13 prefectures previously.

The CDC data documented the 10 cases of anaphylactic shock among a reported 4,041,396 first doses of the Moderna vaccine The Moderna COVID vaccine is pictured. Photo: AFP / Ronny Hartmann