KAWASAKI, Kanagawa Prefecture—A malfunctioning ultracold storage freezer ruined 6,396 COVID-19 vaccine doses, which now must be discarded, the city government here said on June 13.

Kawasaki city said it will use spare doses to avoid a delay in the schedule of its vaccination program.

The vaccine used by the city was developed by U.S. pharmaceutical company Pfizer Inc. and German partner BioNTech, and it must be stored in temperatures of minus 60 to minus 90 degrees.

A city official visiting a storage site for vaccine doses in the city heard an alarm blaring from the freezer in question at 8:10 a.m. on June 13, according to the city government.

Records show the temperature inside the freezer rose to minus 59.6 degrees at 2:08 p.m. on June 11 and hit 9.1 degrees at 2:53 p.m. the following day, the city said. The temperature then began dropping and reached minus 15.2 degrees at 5:05 a.m. on June 13.

The Pfizer vaccine can be stored at 2 to 8 degrees, an average temperature inside regular refrigerators, for one month, but doses that refreeze after thawing cannot be used for inoculations.

Kawasaki city said it decided to dispose of the doses stored inside the freezer because the temperature exceeded 8 degrees at one time.

The freezer was developed by Ebac Co., a Tokyo-based manufacturer of freezers and refrigerators, for exclusive use with the Pfizer vaccine, according to the city.

In a statement released on June 12 on its website, the company confirmed that the temperature inside the same type of the freezer could suddenly rise. It said it will recall 294 units of the freezer it had delivered to its clients and replace them with new ones as soon as possible.