The first of many freezer-packed COVID-19 vaccine vials made their way to distribution sites across the United States on Sunday, 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 FDA, ushers in the biggest vaccination effort in U.S. 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 Monday. Quick transport is key for the vaccine, especially since this one must be stored at extremely low temperatures — about 94 degrees below zero.