"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", Perna said.
Marks also said the FDA is comfortable warning only those with a history of allergic reactions to the Pfizer vaccine's ingredients.
The vaccine has been shown in a clinical trial of 44,000 people to be 95 per cent effective in preventing Covid-19, and no serious safety concerns have been identified.
"People with allergies to any of the ingredients of the vaccine are now cautioned against receiving it", Health Canada said.
With the green light, the USA became one of just a handful of countries - including the United Kingdom and Canada - to allow widespread distribution of the two-dose vaccine, which clinical trials have shown to be 95% effective at preventing the coronavirus.
The first Emergency Use Authorization (EUA) was issued to the COVID-19 vaccine of American drugmaker Pfizer in partnership with German company BioNTech, allowing the vaccine to be distributed in the United States.
On Saturday, FDA commissioner Stephen Hahn denied this had occurred, telling reporters: "The representations in the press that I was threatened to be fired if we didn't get it done by a certain date is inaccurate".
The FDA, in clearing the vaccine Friday, issued a broad authorization for people 16 and older.
Safety monitoring of COVID-19 vaccines was as robust as for other vaccines and the EMA regularly spoke with manufacturers about safety studies and whom to include in clinical trials, the agency also assured.
The first push will vaccinate about three million people.
Each state has the power to determine which groups get priority for limited early supplies of the vaccine, although many governors and state health agencies are looking to the recommendations of a Centers for Disease Control and Prevention advisory panel.
Perna underscored Saturday that the rollout of the vaccine will likely have hitches.
However, residents of long-term care facilities, for example, are already in contact with workers and they can be vaccinated, even if they've been exposed.
On Nov. 21, the United States hit a landmark 12 million cases.
The Pfizer-BioNTech vaccine contains a small piece of the SARS-CoV-2 virus's mRNA that instructs cells in the body to make the virus's distinctive "spike" protein.
And a third vaccine developed with US government assistance, from Sanofi and GlaxoSmithKline, won't be ready until late next year, the companies said Friday - blaming a preliminary study that found an insufficient immune response in older adults.
Both sites will receive 3,000 doses of the vaccine but will only administer 1,500 doses to health-care professionals.
However, in light of their adverse reactions, Health Canada said they had followed up on the reports. The doses are meant to prepare for emergencies, like vaccine spoilage. The Trump administration has previously promised that it will have 20 million doses of the vaccine available by the end of the month.