Some of the first doses of the Pfizer Inc.-BioNTech SE COVID-19 vaccine were held up from delivery in the U.S. this week and sent back to the company because they were colder than anticipated.

Gustave Perna, the army general who serves as Operation Warp Speed’s chief operations officer, said that two trays of vials at two California locations was colder than they are supposed to be. The same thing happened at one location in Alabama, Perna said at a news briefing Wednesday.

Each of the four trays could likely be used to vaccinate 975 people. Pfizer has said its formula needs to be stored at 70 degrees below zero Celsius, the equivalent of negative 94 degrees below zero Fahrenheit. About 2.9 million doses are being distributed across the U.S. this week

The four trays never left the trucks they arrived on, Perna said, adding “we were taking no chances.”

Pfizer, the Food and Drug Administration and the Centers for Disease Control and Prevention are all working to determine whether the formula can still be used when it reaches such low temperatures, according to Perna.

Pfizer did not immediately respond to a request for comment.