Over the weekend, the US became the sixth country to green-light the Pfizer vaccine, following nations like Britain and Canada.
As trucks rolled out of a Pfizer facility in Kalamazoo, Michigan, they were escorted to a local airport by armed US officers.
Two major package delivery services -- UPS and FedEx -- will then ship the supplies to 636 sites around the country by Wednesday.
Some 2.9 million doses of the Pfizer vaccine are being shipped in boxes containing dry ice that can keep supplies at -70 degrees Celsius, the ‘sub-Arctic’ temperature needed to preserve the drug.
The Pfizer vaccine has been shown to be 95 percent effective in preventing Covid-19 infection compared to a placebo. But the FDA has advised people who have known allergies to the vaccine's ingredients to avoid getting it.
Health care workers and nursing home residents will be among the first to receive the Pfizer-BioNTech shots, though it will likely be months before all those who want it can be vaccinated, officials said.
The vaccine is being allocated to states based on each state's adult population. The states then decide the specifics of how to distribute the drug, but are expected to follow the federal guidance to place healthcare workers and nursing home residents at the front of the line.
More than one-third of US deaths have occurred in long-term care facilities.
More than 1.1 million new cases have been confirmed in the past week and the death toll is nearing 298,000 in the US, which has reported the highest death toll and number of cases in the world.
Even as health experts welcomed the unprecedented effort -- with the vaccine developed, tested and distributed in record time -- they cautioned Americans not to grow lax in observing normal precautions.
"The next number of weeks are going to be hell, I fear," New Jersey Governor Phil Murphy told ABC's "This Week."
"So we're begging people to please, please, please don't let your guard down."
Health officials have mounted an education campaign to persuade large numbers of sceptics that the vaccine is safe.
"The way we see light at the end of the tunnel, the way we get through this, is to achieve herd immunity, and that means we need to vaccinate a significant number of people," US Food and Drug Administration commissioner Stephen Hahn said on Sunday on CNN.
(with wires)
