The first trucks carrying a COVID-19 vaccine for widespread use in the US 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 US.

With numbers likely to get worse over the holidays, the vaccine is offering a bright spot in the fight against the pandemic that’s killed nearly 300,000 Americans.

Federal officials say the first shipments of Pfizer’s vaccine will be staggered, arriving in 145 distribution centers Monday, with an additional 425 sites getting shipments Tuesday, and the remaining 66 on Wednesday. The vaccine, co-developed by German partner BioNTech, is being doled out based on each state’s adult population.

Pennsylvania health care giant UPMC has chosen staff who are critical to operating its facilities as among those getting the first round of vaccinations, said Dr. Graham Snyder, who led the center’s vaccine task force.

“It’s very exciting. I will be thrilled, that moment when we administer our first dose,” Snyder said Saturday. “That will clearly be a watershed moment for us.”

Snyder said the UPMC system estimates that half its employees are willing to get the vaccine as soon as it’s offered to them.

The vaccine is heading to hospitals and other sites that can store it at extremely low temperatures – about 94 degrees below zero. Pfizer is using containers with dry ice and GPS-enabled sensors to ensure each shipment stays colder than the weather in Antarctica.

Doses should be delivered to all vaccination sites identified by states, such as local pharmacies, within three weeks, federal officials said.