The first three million doses of the vaccine would be shipped “across all states” this weekend, said Gen Gustave Perna, who is overseeing distribution.
The vaccine offers up to 95% protection against Covid-19 and was deemed safe by the Food and Drug Administration (FDA).
On Saturday, the US recorded a daily toll of 3,309 Covid-related deaths.
The figure, reported on the Johns Hopkins University website, is the highest total in a single day anywhere in the world.
Coronavirus deaths have been rising sharply since November in the US.
Authorising the emergency use of the Pfizer/BioNTech vaccine on Friday, the FDA – which had come under intense pressure from the Trump administration to do so – said the move was a “significant milestone” in the pandemic.
A mass inoculation drive using doses of the same vaccine has already begun in the UK.
During a news conference on Saturday, Gen Perna – speaking for the US government’s vaccination campaign Operation Warp Speed – said doses of the vaccine would be packed into shipping containers for transportation “within the next 24 hours”.
“Expect 145 sites across the states to receive the vaccine on Monday, another 425 sites on Tuesday, and the final 66 sites on Wednesday,” he said, adding that next week’s distribution would complete the initial delivery of the Pfizer/BioNTech vaccine and cover about three million people.
Gen Perna told reporters he was “100% confident” that the doses “needed to defeat the enemy Covid” would be transported safely.
He warned, however, that while it had been a week of progress, “we are not done until every American has access to a vaccine”.
The Pfizer vaccine has already received regulatory approval in the UK, Canada, Bahrain and Saudi Arabia. Like those countries, US health authorities are expected to prioritise health workers and care home residents for the first doses.
More Americans outside the highest-priority groups are likely to be able to get the vaccine in January, with general availability expected by April.
What has the FDA said about the vaccine?
“The FDA’s authorisation for emergency use of the first Covid-19 vaccine is a significant milestone in battling this devastating pandemic that has affected so many families in the United States and around the world,” the head of the agency, Stephen Hahn, said.
He said the authorisation came after “an open and transparent review process” that ensured the vaccine met the “FDA’s rigorous, scientific standards for safety, effectiveness, and manufacturing quality”.
On Thursday, medical experts advising the FDA recommended the emergency-use authorisation. A 23-member panel concluded the vaccine’s benefits outweighed its risks.
Emergency use, the FDA said, was not the same as full approval, which would require Pfizer to file a separate application to secure.
US media reported that Mr Hahn had earlier been told to approve the vaccine for emergency use by Friday or quit.
Mr Hahn, however, said the reports were “untrue” and stressed that the agency had not compromised safety in its testing.
How does the vaccine work?
The Pfizer/BioNTech vaccine was the first coronavirus jab to show promising results in the latter stages of its testing process.
It is a new type called an mRNA vaccine that uses a tiny fragment of genetic code from the pandemic virus to teach the body how to fight Covid-19 and build immunity.
“The vaccine contains a small piece of the [Covid-19] virus’s mRNA that instructs cells in the body to make the virus’s distinctive ‘spike’ protein,” the FDA said.
“When a person receives this vaccine, their body produces copies of the spike protein, which does not cause disease, but triggers the immune system to learn to react defensively, producing an immune response against [Covid-19].”
The vaccine is given as two injections, 21 days apart, with the second dose being a booster. Immunity begins to kick in after the first dose but reaches its full effect seven days after the second dose.
The vaccine must be stored at ultra-low temperatures, which makes distribution difficult. Special shipping containers that use dry ice will be used to transport frozen vials direct to the point of vaccination, Pfizer says.
The pharmaceutical company has agreed a deal to supply the US with 100 million doses of the vaccine by March.
An additional 200 million doses of a second vaccine, developed by Moderna and the National Institutes of Health, will be provided by June. However this vaccine is still seeking approval in the US.