The federal government plans to send 6.4 million doses of pharmaceutical giant Pfizer’s coronavirus vaccine to communities across the United States within 24 hours of regulatory clearance, with the expectation that shots will be administered quickly to front-line health-care workers, the top priority group, officials said Tuesday.

Gen. Gustave Perna, who oversees logistics for Operation Warp Speed, the Trump administration’s effort to speed up treatments and vaccines, told reporters that state officials were informed Friday night of the allocation, which is based on each state’s overall population.

The amount would cover a portion of the nation’s 20 million health-care workers, let alone the U.S. population of 330 million. But Perna said “a steady drumbeat” of additional doses will be delivered as manufacturing capacity ramps up in each successive week.

With increased prospects that federal regulators will authorize the Pfizer vaccine on an emergency basis as early as mid-December, and the first shots administered before the end of the year, Operation Warp Speed has begun to release more details about the massive and complicated distribution effort to immunize tens of millions of Americans.

U.S. government officials are on track to have 40 million doses of vaccines from Pfizer and a second company, biotech firm Moderna, by the end of the year, enough to vaccinate 20 million people. (Each vaccine requires two doses). It is likely to be April before the general public begins to get vaccinated.

The initial 6.4 million doses also includes vaccines that would go to five federal agencies – the Bureau of Prisons, the Defense and State departments, Indian Health Service, and the Veterans Health Administration – that receive allocations directly from the federal government.

States and territories now have the necessary information to “plan and figure out where they want the vaccine distributed” in the first shipment, Perna said. States are supposed to designate their top five sites capable of receiving and administering the Pfizer vaccine, which must be stored at ultracold temperatures of minus-70 Celsius (minus-94 Fahrenheit), and has exacting handling protocols. The ultracold temperature is significantly below the standard for most vaccines of 2-8 degrees Celsius (36-46°F).

Many states have designated large hospital systems to be the first places to receive vaccines because they have ultracold freezers and can efficiently vaccinate many people. The minimum order for the Pfizer vaccine is 975 doses; for Moderna’s, with a storage temperature that does not require such freezers, the minimum order is 100.

Once a vaccine is cleared by the FDA, an independent advisory panel to the Centers for Disease Control and Prevention – the Advisory Committee on Immunization Practices – will hold a public meeting within 48 hours to vote on final recommendations for the vaccine’s use and who should get the first shots. Health-care workers will be the first priority, the group has said. About 3 million residents of long-term care facilities are also likely to be included in that first phase. Next in line will be an estimated 87 million other essential workers, including first responders, teachers and grocery workers; more than 100 million adults with high-risk medical conditions; and about 53 million adults over the age of 65.

Within 24 hours of FDA action, doses will be “propositioned” at the sites designated by each state to give the shots to the first groups.

Pfizer has been conducting dry runs of each step, from vaccine delivery to opening Pfizer’s GPS-tracked special containers to vaccine storage, Perna said. The company began working last week with four states – Rhode Island, New Mexico, Tennessee and Texas – to familiarize personnel with storage and handling requirements. These dry runs do not include actual vaccines or the dry ice that will be used to keep the vials cold. Additional rehearsals in coming weeks will include dry ice, a federal health official said, speaking on the condition of anonymity because the person was not authorized to speak publicly.

Those “lessons learned” are being shared with other officials, Perna said. There was “initial hesitation” from some personnel at the sites, he said, but “we expect to see growing confidence in people that are using it.”

Americans will receive the vaccine free. The federal government is paying for much of the delivery and vaccine administration costs. But funding remains a big issue for state and local officials, who are asking Congress for at least $8 billion for vaccination efforts; to date, $200 million in federal funds has been sent to state, territorial and local jurisdictions to help them prepare. Federal officials are sending another $140 million in December.

Jeff Duchin, a top official at the Seattle and King County health department, said the more than $10 billion in taxpayer dollars spent on development of covid-19 vaccines by Operation Warp Speed was appropriate.

“But it’s been more like Operation Status Quo with respect to providing the federal funding needed for state and local health departments to actually get vaccine to the population, including the initial priority populations and ultimately, to as many people as possible,” he said in an email Tuesday.

State and local officials say much of the critical planning and implementation work needed for distribution is not adequately funded or staffed. That work includes planning with a broad range of health-care providers for the necessary training and upgrading information systems to vaccinate hard-to-reach and undeserved populations, Duchin said.

Health-care providers also need to track allocations and vaccinations administered, and ensure that people come back for second doses. Public health officials also need to do outreach with local communities that are hesitant about getting the vaccine, he said.

“Tens of millions of dollars are needed for this work in our county and state,” he said. “In addition, this work is tasked to local and state public health departments and workers, who have been grappling with this pandemic nonstop for months and are running on fumes.”