The nation’s first COVID-19 vaccine will begin arriving in states Monday morning, U.S. officials said Saturday, after the government gave the final go-ahead to the shots needed to end an outbreak that has killed nearly 300,000 Americans.

Trucks will roll out Sunday morning as shipping companies UPS and FedEx begin delivering Pfizer’s vaccine to nearly 150 locations, said Army Gen. Gustave F. Perna of Operation Warp Speed, the Trump administration’s vaccine development program. Another 450 sites will get the vaccine Tuesday and Wednesday.

The locations include hospitals and other sites able to meet the ultra-cold storage requirements for the vaccine. Within three weeks, vaccines should be delivered to local pharmacies and other locations, Perna said at a news conference.

The vaccine was timed to arrive Monday morning so that health workers would be available to receive the shots and begin giving them, Perna said.

It was unclear who would receive the first dose of the vaccine, though health workers and nursing home residents were the priority. Perna said that decision would be determined by health authorities.

The announcement kicks off a massive logistical operation involving the federal and state governments, private companies and health care workers to quickly distribute limited vaccine supplies throughout the U.S.

Initially, about 3 million shots are expected to shipped nationwide, according to officials with Operation Warp Speed. A similar amount is to be held in reserve for those recipients’ second dose.

Initial shipments are expected to leave Pfizer’s manufacturing plant in Kalamazoo, Michigan, via truck and then be flown to regional hubs around the country.

Adding to the distribution challenge is that the Pfizer-BioNTech vaccine must be stored and shipped at ultra-low temperatures of about 94 degrees below zero Fahrenheit. Pfizer has developed shipping containers that use dry ice, and GPS-enabled sensors will allow the company to track each shipment and ensure it stays cold.

The green light to begin vaccinations came late Friday after the Food and Drug Administration authorized emergency use of the vaccine. The signoff capped an unprecedented global race to speed vaccines through testing and review, chopping years off the normal development process.

The FDA found the vaccine highly protective with no major safety issues. U.S. regulators worked for months to emphasize the rigor and independence of their review, but the Trump administration pressured the agency up until the final announcement. A top White House official even threatened to remove FDA chief Stephen Hahn if a ruling did not come before Saturday.

Concerns that a shot was rushed out could undermine vaccination efforts in a country with deeply ingrained skepticism about vaccines. Hahn again emphasized his agency’s independence to reporters Saturday.

“Science and data guided the FDA’s decision,” Hahn said. “We worked quickly because of the urgency of this pandemic, not because of any other external pressure.”

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

The FDA’s vaccine director, Dr. Peter Marks, said the agency will carefully track any reports of allergic reactions in the U.S.

“I think we still need to learn more, and that’s why we’ll be taking precautions,” Marks said.

The FDA next week will review a second vaccine from Moderna and the National Institutes of Health that appears about as protective as Pfizer’s shot. On Friday, the Trump administration announced it had purchased 100 million more doses of that vaccine on top of 100 million it previously ordered.

The announcement came after revelations last week that the White House opted not to lock in an additional 100 million doses of Pfizer’s vaccine for delivery in the second quarter of 2021. The Trump administration contends the current orders plus those in the pipeline will be enough to accommodate any American who wants to be vaccinated by the end of the second quarter of 2021.

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Associated Press writer Candice Choi contributed to this story from New York.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all contentcontent.

This is the first winter in five years that Steve Monk and his wife, Linda, haven’t driven to Arizona from their home in Prince Albert, Saskatchewan.

They typically leave Canada to hunker down in warmer climates for six months. They could fly, skirting travel restrictions at the border, but they’d rather “freeze their buns off” than go to the U.S., where COVID-19 infections and deaths are surging.

“It’s not worth taking a chance. It’s not nearly as bad in this country as it is down there,” said Monk, 69. “Pretty much every Canadian person we do know that goes down (to the U.S.) is not going. It’s pretty widespread.”

Snowbirds like the Monks, often retirees who live somewhere warm like Arizona or Florida part time to escape cold weather, won’t be flocking south this winter. For Canadians who drive, nonessential border travel is banned until at least Dec. 21. For some, it’s fear of the virus.

While their absence is being felt by vacation rentals, restaurants and shops, RV parks and campgrounds are seeing an increase in campers as people travel closer to home.

A huge chunk of the snowbird population is Canadian. Evan Rachkovsky of the Canadian Snowbird Association said most people he’s spoken with are suspending trips to the U.S.

But some are still adamant about going.

“Some tell me just simply this is something they’ve been doing for 10, 20, 30 years, so it’s habitual in that sense,” Rachkovsky said. “It’s a lifestyle as opposed to vacationing for two weeks.”

For those who go, they may face recommendations to quarantine for up to two weeks, though states often don’t enforce it. They’re also going into communities where hospitals are normally busiest during the winter months, and COVID-19 could overwhelm them.

Health insurance hurdles are deterring retired Toronto accountant Mel Greenglass, who for almost a decade has spent four months in southwest Florida near Naples. Canadian snowbirds must buy a supplemental plan to their government-provided coverage for any emergencies during their stay. It would have been $2,800 for him and his girlfriend this season, up from $1,800 previously, and he feared they wouldn’t be covered if they caught the virus.

Insurers “are not going to lay out a lot of money to cover everybody just by raising their premiums a little bit,” said Greenglass, 78. He added that adapting to the Canadian winter won’t be easy: “I don’t even own a pair of boots.”

It’s easier for those who don’t have international borders to cross. Kathy Scott, 73, and her 81-year-old husband intend to make their annual drive from Salt Lake City to Arizona after Christmas.

Scott said she plans to mask up and practice social distancing to avoid burdening the medical system, adding that she’s “not having any problem asking people about having been tested, about quarantining, about where they’ve been.”

Snowbirds’ plans have a huge impact on tourism. In Florida, 3.6 million Canadians visited last year, making up a quarter of its foreign tourists, according to the state tourism office. Visit Florida estimates that only 15,000 Canadians arrived between April and September, the last month with available statistics. That’s about an 99% decrease from the same period last year.

The Arizona Office of Tourism said an estimated 964,000 Canadian visitors were responsible for $1 billion of the $26.5 billion in tourism spending last year. In September, visitors overall spent $752 million, down 60% from the $1.9 billion expected in a normal year.

Becky Blaine, the office’s deputy director, said it helps that many people are looking closer to home for vacation. But that will only go so far to offset the loss of international visitors. She’s also not sure how much of a boost RV parks and campgrounds will get.

“Now that kids are back in school though, it would be more of that retiree population versus over the summer when everybody rented RVs, including myself,” Blaine said.

Bobby Cornwell, executive director of the Florida and Alabama RV Parks & Campground Association, believes it’s not “all doom and gloom” for his industry. Snowbirds make up 30% of the business for Florida’s RV parks, he said. There have been cancellations, but park operators are seeing people of all ages road-tripping.

“I really wanna hammer this home: From the people we’re getting feedback from, many of our parks throughout the whole state, for every cancellation, there’s one or two campers who come in,” Cornwell said. “I haven’t heard of anything disastrous.”

Bruce Hoban, co-founder of the 2,000-member Vacation Rental Owners and Neighbors of Palm Springs, said property managers who rent condos to snowbirds for two- to three-month stints in the desert resort city are having a hard time. But vacation rentals for stays under 30 days have been “through the roof.”

Normally, rentals generate 25% of the $25 million Palm Springs rakes in from an occupancy tax. They’re now generating 50%. Between vacation rentals and hotels, the city did 5.5% more business between July and September compared with the same period last year.

“It’s a big shift,” Hoban said. “The amount of people coming on vacation rentals was like nothing we had ever seen. … Yes, we lost two-and-a-half months of what is normally our most expensive, highest time of the year because of Coachella festivals and stuff. We lost all that. We have more than made up for it since then.”

But shop owners like Julie Kathawa, 49, aren’t expecting big business from younger vacationers. Julie’s Hallmark sells cards and gifts in Bermuda Dunes, outside Palm Springs, and already feels the crunch of fewer snowbirds, who make up about 20% of her business from November to April. She’s relying mostly on online mail orders.

“I’m grateful for it, but it’s not the same. I think it’s going to help me through December,” Kathawa said. “Other than that, I don’t think it’s going to be as profitable or as exciting in January, February and March because we would still have all this tourism.”