The first of many freezer-packed Covid-19 vaccine vials have made their way to distribution sites across the United States, as the nation’s pandemic deaths approached the horrifying new milestone of 300,000.

The rollout of the Pfizer vaccine, the first to be approved by the Food and Drug Administration, ushers in the biggest vaccination effort in US history — one that health officials hope the American public will embrace, even as some have voiced initial skepticism or worry. Shots are expected to be given to health care workers and nursing home residents beginning today.

According to AP, quick transport is key for the vaccine, especially since this one must be stored at extremely low temperatures — about 94 degrees below zero.

Tracked with GPS-enabled sensors, the initial shipments were expected to contain about three million doses, with many more to come. Federal officials say the first shipments of Pfizer’s vaccine will be staggered, arriving in 145 distribution centers today, with another 425 sites getting shipments tomorrow, and the remaining 66 on Wednesday.