Less than 24 hours after Moderna's COVID-19 vaccine was authorized by the FDA, Operation Warp Speed's chief operating officer Gen. Gus Perna said the second vaccine to be approved in the U.S. is being packed and loaded and will start rolling out Sunday.
Perna said Saturday that amid snowstorms and the holiday shipping rush, 2.9 million doses of the previously-authorized Pfizer COVID-19 vaccine have been delivered over the last week across the country, to every state.
He expects in the coming week, with both the Pfizer and Moderna vaccines going out for distribution, there will be 7.9 million doses delivered across 3,700-plus locations, including hospitals, doctor offices and pharmacies.
In addition to private companies like Pfizer, Moderna, FedEx and UPS, the OWS collaboration includes the Centers for Disease Control and Prevention, the National Institutes of Health, the Biomedical Advanced Research and Development Authority and the Department of Defense.
Perna says he feels "confident" about their goal of processing 20 million vaccines by the end of December and finishing distribution across the U.S. by the first week of January.
Perna also addressed allegations that vaccine doses were cut, explaining he took complete personal responsibility in approving a larger number of vaccines that were actually releasable, in part because of the FDA's rigorous checks for safety, as well as a non-stop production line from manufacturing to processing to distribution.
He said he has spoken to governors from several states to personally apologize for the error.
"At the end of the day, this is all about enabling the governors and the states to ensure that their people receive the vaccine in a fair and equitable process -- and that when they receive it, they can have confidence that the vaccine is safe and ready to be administered in their arms," Perna said.
On Friday, a board of independent advisers overwhelmingly voted to recommend the Moderna vaccine for people over the age of 18. Shortly after, the U.S. Food and Drug Administration approved the vaccine, triggering the shipping of 5.9 million doses.
The Pfizer and Moderna vaccines are similar in several ways, including using messenger RNA technology, and both have been shown to be more than 94% effective in protecting against COVID-19 across race, gender and underlying medical conditions.
Side effects for both vaccines have been shown to be mild and temporary, including symptoms of pain at the injection site, headache, fever, fatigue, chills and muscle and joint pain.
The two vaccines have subtle differences, but it will be ultimately unlikely Americans will be able to choose which shot they get as it will depend on which is available in their area.
The two differ in part because Moderna's vaccine can be kept in a conventional freezer at minus 4 degrees, while Pfizer's requires a special freezer to maintain a minus 94 degree environment.
While they both have two-dose timelines, Moderna's is slightly longer at a 28-day schedule between shots, and Pfizer's is 21 days.
The companies' vaccines are also authorized for slightly different age groups. Pfizer's vaccine is authorized for people 16 years old and up, while Moderna's authorization request includes people 18 and older.
A third vaccine also is on the horizon. Johnson & Johnson was expected by early January to know whether its vaccine was effective. If that vaccine comes online as well, that "will help us accelerate even faster coverage of that population," Moncef Slaoui, Trump's top science adviser in the vaccine effort, told CNBC.
ABC News' Erin Schumaker contributed to this report.