On Sunday, the first lot of many COVID-19 vaccine vials made their way to distribution sites across the United States, as the virus continues to kill more and more Americans, with the officials reporting more than 306,000 deaths.

The Pfizer-BioNTech vaccine is the first to get approval from the Food and Drug Administration (FDA). The rollout of vaccines ushers in the biggest vaccination effort in the history of the United States.

The first shot of the vaccine is expected to be given in this week to health workers and long-term nursing home residents.

Early Sunday, Pfizer workers wasted no time in packing vials into boxes, scanning the packages, and then placing them into freezer cases with dry ice.

Richard Smith of FedEx Express said, “This is a historic day.” The American multinational delivery services company is delivering more than 630 packages of the vaccine to distribute across the nation. Also, the United Parcel Service is transporting a share of the vaccine.

The initial shipments were expected to contain nearly 3 million doses, with additional doses to come. The US government officials said the first shipments of Pfizer’s COVID vaccine will be staggered, arriving in 145 distribution centers today, with additional 425 sites getting shipments by Tuesday, while the remaining 66 sites on Wednesday.

The vaccine doses will be delivered based on each state’s adult population and then the states will decide where they go first.

State health departments have been prioritizing hospitals with adequate storage facilities and the ones that serve high-risk populations.

Dr. Sandra Kemmerly of Oschner Health System in Louisiana and Mississippi said, “There’s that thought that maybe they [health workers] don’t have to be so afraid to come to work if they can be vaccinated and be immune.”

“Employees approved for the first round are getting texts and emails directing them to schedule their initial injection,” she added.

Nearly half of the Americans want to get vaccinated as soon as possible, according to a survey from The Associated Press-NORC Center for Public Affairs Research. Another quarter is not sure, while the others say they are not interested in getting vaccinated.

Some Americans simply oppose getting immunized in general, while others express concerns over rushing the vaccines too soon and want to see how the rollout goes.

FDA Commissioner Dr. Stephen Hahn has repeatedly insisted that the agency’s decision was based on science, not politics, despite a threat from the White House to fire him if the vaccine was not approved before Saturday.

Chief Science Adviser to Operation Warp Speed Dr. Moncef Slaoui told Fox News that he is “very concerned” about the skepticism about the vaccine in some circles.

“Unfortunately, there’s been a confusion between how thorough and scientific and factual the work that has been done is, and the perception that people are thinking that we cut corners,”

  he added. “I can guarantee you that no such things have happened, that we follow the science.”

Dr. Slaoui said the development of vaccines from drug companies, including Moderna and AstraZeneca, is “a remarkable achievement of science, academia, the industry ecosystem and the U.S. government, working together. The article originally appeared on Medical Xpress.