The U.S. will begin delivering initial doses of the Pfizer-BioNTech COVID-19 vaccine starting Monday, according to General Gus Perna, chief operating officer of Operation Warp Speed.

"They will begin moving vaccine from the Pfizer manufacturing facility to the UPS and FedEx hubs, and then it will go out to the 636 locations nationwide, which were identified by the states and territories," Perna said on Saturday.

"We expect 145 sites across all the states to receive vaccine on Monday, another 425 sites on Tuesday, and the final 66 sites on Wednesday, which will complete the initial delivery of the Pfizer orders for vaccine."

Initially, about 3 million doses were expected to be shipped nationwide. It was unclear exactly who would receive the first shots, though healthcare workers and nursing home residents were the priority. Perna said health authorities would decide.

The development comes after the U.S. Food and Drug Administration (FDA) on Friday authorized Pfizer-BioNTech COVID-19 vaccine for emergency use. 

With the emergency clearance, the U.S. becomes the latest country to authorize the Pfizer-BioNTech vaccine. The vaccine was first approved in Britain earlier this month.

The FDA said the vaccine can be given to people aged 16 and older. Healthcare workers and elderly people in long-term care facilities in the U.S. are expected to be the main recipients of a first round of 2.9 million doses.

Meanwhile, the White House announced to opt out for 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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The first shipments will leave Pfizer's manufacturing plant in Kalamazoo, Michigan by truck and then be flown to regional hubs around the country. Medical distributor McKesson and pharmacy chains, including CVS and Rite-Aid, are also involved in the initial rollout and vaccinations at nursing homes and assisted living centers.

In a key distribution challenge, the vaccine, co-developed with BioNTech, must be stored and shipped at ultra-low temperatures – about 94 degrees Celsius below zero. 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.

FedEx Executive Vice President Richard Smith told CNN that the delivery services company will be using "SenseAware ID" to track the transportation of the COVID-19 vaccine. The new sensor-based tracking device was launched in September and developed in conjunction with Microsoft.  

"We'll be able to know where they are at all times, we'll have our priority alert agents monitoring them," Smith said.  

The U.S. remains to be the hardest-hit country globally by the pandemic, having recorded at least 16 million cases of coronavirus and 297,501 deaths, according to data compiled by Johns Hopkins University.  

It accounts for 22.25 percent of the world's reported COVID-19 infections and 18.49 percent of the related global fatalities.

Vaccine hesitancy is now the biggest challenge remaining in the fight against COVID-19, Dr. Susan Bailey, president of the American Medical Association, said in a statement Saturday.

"Manufacturing, distribution and administration still pose challenges, but the biggest threat remaining may be people's willingness to get vaccinated," she said. "To be clear, these vaccines will reduce death and severe illness. They have been rigorously evaluated, and if enough of us roll up our sleeves and get vaccinated, we can eventually reclaim normalcy."

(Cover photo: Pfizer's Global Supply Kalamazoo manufacturing plant is shown in Portage, Michigan, U.S., December 11, 2020. /AP)

(With input from agencies)