The first trucks carrying a COVID-19 vaccine for widespread use in the United States were set to pull out of a Michigan manufacturing plant Sunday, with the shots that are critical to stopping the nation's coronavirus outbreak destined to reach states a day later.Shipments of the Pfizer vaccine will set in motion the biggest vaccination effort in American history at a critical juncture of the pandemic that has killed 1.6 million and sickened 71 million worldwide.Initially, about 3 million doses were expected to be sent out, and the priority is health care workers and nursing home residents as infections, hospitalizations and deaths soar in the U.S.