The U.S. surpassed 300,000 coronavirus deaths on Monday as the first of many freezer-packed COVID-19 vaccine vials made their way to distribution sites across the United States.

According to Johns Hopkins University’s School of Medicine, the U.S. continues to lead the world in the number of confirmed cases — 16.4 million — and deaths, ahead of Brazil’s more than 181,000. The U.S. now has recorded 300,420 deaths, according to Johns Hopkins data, as of 6:45 p.m. ET Monday.

“The numbers are staggering — the most impactful respiratory pandemic that we have experienced in over 102 years, since the iconic 1918 Spanish flu,” Dr. Anthony Fauci, the government’s top infectious disease expert, said days before the milestone.

On Monday morning, New York intensive care nurse Sandra Lindsay became the first person in the state — and possibly the nation — to receive the new Pfizer-BioNTech coronavirus vaccine.

The rollout of the Pfizer vaccine, the first to be approved for emergency use authorization by the Food and Drug Administration, ushers in the biggest vaccination effort in U.S. history. Shots are expected to be given to health care workers and nursing home residents beginning Monday.

Hospital workers began unloading frozen vials of COVID-19 vaccine Monday. Packed in dry ice to stay at ultra-frozen temperatures, the first of nearly 3 million doses being shipped in staggered batches this week made their way by truck and by plane around the country Sunday from Pfizer’s Kalamazoo, Michigan, factory. Once they arrive at distribution centers, each state directs where the doses go next.

Some hospitals across the country spent the weekend tracking their packages, refreshing FedEx and UPS websites for clues.

More of the Pfizer-BioNTech vaccine will arrive each week. And later this week, the FDA will decide whether to green-light the world’s second rigorously studied COVID-19 vaccine, made by Moderna Inc.

Now the hurdle is to rapidly get vaccine into the arms of millions, not just doctors and nurses but other at-risk health workers such as janitors and food handlers, and then deliver a second dose three weeks later.

The FDA, considered the world’s most strict medical regulator, said the Pfizer-BioNTech vaccine appears safe and strongly protective, and laid out the data behind it in a daylong public meeting last week for scientists and consumers alike to see.

“Please, people, when you look back in a year and you say to yourself, ‘Did I do the right thing?’ I hope you’ll be able to say, ‘Yes, because I looked at the evidence,’” Dr. Francis Collins, director of the National Institutes of Health, said Sunday on NBC’s “Meet the Press.” “People are dying right now. How could you possibly say, ‘Let’s wait and see.’”

Still, emergency use means the vaccine was cleared for widespread use before a final study in nearly 44,000 people is complete, and that research is continuing to try to answer additional questions. While effective against COVID-19 illness, it’s not yet clear if vaccination will stop the symptomless spread that accounts for half of all cases.

The shots still must be studied in children and during pregnancy. But the American College of Obstetricians and Gynecologists said late Sunday that vaccination should not be withheld from pregnant women who otherwise would qualify.

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.

2 UK health workers suffer allergic reaction to COVID-19 vaccine

Quick transport is key for the vaccine, especially because this one must be stored at extremely low temperatures — about 94 degrees below zero. Early Sunday, workers at Pfizer — dressed in fluorescent yellow clothing, hard hats and gloves — wasted no time as they packed vials into boxes. They scanned the packages and then placed them into freezer cases with dry ice. The vaccines were then taken from Pfizer’s Portage, Michigan, facility to Gerald R. Ford International Airport in Grand Rapids, where the first cargo plane took off amid what airport officials called a “jubilant” mood.

“This is a historic day,” said Richard W. Smith, who oversees operations in the Americas for FedEx Express, which is delivering 630-some packages of vaccine to distribution sites across the country. United Parcel Service also is transporting a share of the vaccine.

Tracked with GPS-enabled sensors, the initial shipments were expected to contain about 3 million doses, with many more to come. Federal officials say the first shipments of Pfizer’s vaccine will be staggered, arriving in 145 distribution centers Monday, with another 425 sites getting shipments Tuesday, and the remaining 66 on Wednesday. Doses of the vaccine, co-developed by German partner BioNTech, are given out based on each state’s adult population. Then the states decide where they go first.

How COVID-19 vaccines were developed so quickly

Dr. Stephen Hahn, commissioner of the FDA, which approved the Pfizer vaccine Friday, has repeatedly insisted that the agency’s decision was based on science, not politics, despite a White House threat to fire him if the vaccine wasn’t approved before Saturday.

Speaking to “Fox News Sunday,” Dr. Moncef Slaoui, chief science adviser to Operation Warp Speed, a U.S. effort to get vaccines developed quickly, also said he is “very concerned” about the skepticism about the vaccine in some circles.