TRACTOR trailers loaded with suitcase-sized containers of COVID-19 vaccine will leave Pfizer Inc's manufacturing facility in Kalamazoo, Michigan, on Sunday morning - launching the largest and most complex vaccine distribution project in the United States, where the virus is raging.

U.S. regulators late on Friday authorized the vaccine from Pfizer and partner BioNTech for use, and U.S. marshals will accompany the tightly secured shipments from factory to final destination.

"We have spent months strategizing with Operation Warp Speed officials and our healthcare customers on efficient vaccine logistics, and the time has arrived to put the plan into action," Wes Wheeler, president of UPS Healthcare, said on Saturday.

Pfizer's dry-ice cooled packages can hold as many as 4,875 doses, and the first leg of their journey will be from Kalamazoo to planes positioned nearby. Workers will load the vaccine - which must be kept at sub-Arctic temperatures - onto the aircraft that will shuttle them to United Parcel Service or FedEx air cargo hubs in Louisville, Kentucky, and Memphis, Tennessee, respectively.

From there, they will be trucked or flown to facilities close to the 145 U.S. sites earmarked to receive the first doses.

Familiar UPS and FedEx package delivery drivers, who may also be carrying holiday gifts and other parcels, will deliver many of the "suitcases" into the hands of healthcare providers on Monday. The shipments are the first of three expected this week.

Healthcare workers and elderly residents of long-term care homes are first in line to receive the inoculations.

Pfizer's inoculations have the most restrictive requirements for shipping and storage temperature, minus 70 degrees Celsius (minus 94°F).

UPS and FedEx are giving the vaccine top priority, reserving space on planes and trucks at a time when pandemic- and holiday- related e-commerce are creating more demand for deliveries than carriers can handle.

Both companies have expertise handling fragile medical products and are leaving little room for error. They are providing temperature and location tracking to backup devices embedded in the Pfizer boxes, and tracking each shipment throughout its journey.