OSNABRUECK, GERMANY - DECEMBER 13: (EDITOR´S NOTE: This image was converted from colour into black and white)A medical worker of the Johanniter, a Protestant aid organization, holds a syringe filled with saline solution during a test run at a future Covid-19 vaccine center on December 13, 2020 in Osnabrueck, Germany. Across Germany event arenas, trade fair halls, gymnasiums and other venues are being converted into vaccine centers, with many beginning test runs ahead of approval by European Union health authorities of the new vaccine. The first real inoculations could begin by the end of December. (Photo by Alexander Koerner/Getty Images)

 The first shipments of a COVID-19 vaccine for widespread use in the United States headed Sunday from Michigan to distribution centers across the country, with the first shots expected to be given in the coming week to health care workers and at nursing homes.

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. 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.

While the vaccine was determined to be safe, regulators in the U.K. are investigating several severe allergic reactions. The FDA’s instructions tell providers not to give it to those with a known history of severe allergic reactions to any of its ingredients.

Another vaccine by Moderna will be reviewed by an expert panel next week and soon afterward could be allowed for public use.