The federal government is asking four states and one city to draft plans for how they would distribute a coronavirus vaccine when limited doses become available, possibly as early as this fall.

The Centers for Disease Control and Prevention, the Defense Department and other agencies began working with officials in California, Florida, Minnesota, North Dakota and Philadelphia this week to develop plans to transport and store vaccine doses, and to prioritize who would receive them.

Those plans, which will take into account each location’s racial and ethnic makeup, population density and other factors, will be shared with other states to help with their own vaccine distribution planning. The discussions with states this week offer some of the first details of the federal government’s plans at a time when information shared by the administration has been limited and often confusing.

The race for a vaccine continues to unfold amid the backdrop of a crippling pandemic that shows few signs of abating.

As of Friday, at least 164,000 Americans have died after becoming infected, according to data tracked by The Washington Post. The seven-day average for deaths remains above 1,000 per day, where it has remained since July 27. On average, more than 50,000 new U.S. cases have been diagnosed per day since July 6. On Friday, California became the first state to report more than 600,000 coronavirus cases.

The United States has begun planning the largest vaccination campaign ever undertaken, requiring extraordinary coordination, planning and communication. U.S. officials said this week that Operation Warp Speed, the Trump administration’s effort to expedite development of coronavirus countermeasures, is on track to deliver tens of millions of vaccine doses by January.

President Trump has repeatedly said he hopes a viable vaccine can be ready before year’s end — a startlingly fast goal compared with the normal process — but public health experts have said the timetable remains in flux.

No vaccine for the novel coronavirus, which causes the disease named covid-19, has yet been approved. But of the roughly 200 experimental vaccines aimed at ending the pandemic, two have moved into the advanced stage of testing in the United States in thousands of people to check their effectiveness and safety.

In planning discussions, one of the hottest topics involves freezers. At least one vaccine candidate is expected to require storage at very cold temperatures, about minus 70 degrees Celsius (minus 94 Fahrenheit).

A top CDC official told state immunization officials Wednesday that states probably won’t be expected to buy special freezers. But if a vaccine is approved by the Food and Drug Administration that requires such cold storage, states should prepare sites for mass vaccination clinics, because doctors’ offices aren’t likely to store and administer such shots.

On Friday, the Defense and Health and Human Services departments announced that McKesson Corp. will be a central distributor of the vaccines and related supplies. The CDC is executing an existing contract option of $178 million with McKesson to support vaccine distribution, an HHS statement said. McKesson also distributed the H1N1 vaccine during that pandemic in 2009-2010. The company will work under the CDC’s guidance to ship the vaccines to sites where shots will be administered, the statement said.

Although Trump has said repeatedly that the military will deliver vaccines, the Defense Department “is not actually going to be distributing or delivering the vaccines itself,” Paul Mango, deputy chief of staff for policy at HHS, told reporters this week. Rather, the military will handle logistics of manufacturing, including acquisition of raw material, establishing factories and training workers.

“With few exceptions, our commercial distribution partners will be responsible for handling all the vaccines,” Mango said in an email Thursday.

In the months ahead, state and federal officials will face a huge logistical challenge. They must figure out how to transport and store massive amounts of vaccine. They must also determine who should get the first doses.

The first doses will probably be given to high-priority groups such as front-line health-care personnel and essential workers. Final recommendations on who is considered high priority are expected to be made this fall by an independent advisory committee and a federal immunization advisory panel.

In a letter dated Aug. 4 and sent to state officials, the CDC directed states to make several assumptions for their planning.

States should assume any vaccine will be distributed directly to health-care providers. Vaccine providers must enroll with their jurisdiction’s immunization program to receive doses. Needles and syringes and limited amounts of face masks and face shields will be distributed to providers “proportionately by the federal government at no cost,” the letter to states said.

The amount of vaccine allocated to each jurisdiction will be based on several factors, including population size, the letter states.

Recommendations on which groups should receive vaccines will probably change, depending on the characteristics of each vaccine, available supply and the disease’s epidemiology.

In a meeting with state immunization officials and other experts Wednesday, Nancy Messonnier, who leads the CDC’s National Center for Immunization and Respiratory Diseases, stressed the need for urgency and flexibility in completing plans by Oct. 1, the earliest she said vaccines could be available.

“We need states to have draft plans even if the draft plans aren’t perfect,” she told the group.

Other U.S. officials have said such an October scenario is extremely unlikely.

“That would be astounding,” National Institutes of Health Director Francis Collins said during a Thursday HHS briefing. The only way that could happen, he said, is if one of the advanced trials underway was able to enroll volunteers “at absolute record speed,” and the vaccine was deemed safe and effective by the FDA even before all the volunteers signed up.

“That’s a number of unlikelihoods piled on top of each other,” Collins said, adding, “Maybe November, December would be my best bet.”

The relentless nature of the pandemic has battered not only the nation’s economy and those on the front line of combating the virus, but also a large swatch of ordinary Americans.

A report from the CDC on Friday found that 41 percent of more than 5,400 respondents to a recent Web-based survey reported “considerably elevated adverse mental health conditions associated with covid-19,” including the 30 percent who outlined symptoms of anxiety and depression.

At least 13 percent of those who responded with mental health symptoms reported having started use or increasing use of controlled substances, and nearly 11 percent said they had seriously considered suicide in the previous 30 days.

Hispanic respondents had a higher prevalence of anxiety disorder and depressive disorder symptoms, substance abuse and suicidal thoughts than non-Hispanic Whites or Asians. The report noted that Black respondents also saw an increase in substance use and suicidal thoughts in the past 30 days more than Whites and Asians who participated.

Responses to the pandemic have also acutely affected essential workers and unpaid caregivers, according to the report, which concluded that the identified high-risk groups should be prioritized in intervention and prevention efforts.

Friday also brought the latest reminders that the virus is adept at spreading wherever people gather, as the rocky reopening of schools across the country has demonstrated.

Less than a week into the start of classes at the University of North Carolina at Chapel Hill, school officials on Friday announced two clusters of coronavirus cases in student housing.

The university said in an alert sent to students and staff that the clusters, which are five or more cases within the same buildings, were identified at Ehringhaus Community, a freshman dorm, and Granville Towers apartments.

The school didn’t say how many infections are part of the outbreaks. A spokesperson didn’t immediately respond to a request for comment from The Washington Post.

The news of the clusters comes as other schools bring back students and report surges in cases. The University of Notre Dame tallied 29 confirmed cases Friday — more than double its count two days ago, according to a schoolwide tracker.

One of the main concerns for students, parents and administrators is the spread of the virus in student housing, especially in communal spaces or where residents may share close contact.

Columbia University President Lee C. Bollinger announced Friday that the school will “drastically scale back” student housing, meaning all classes will need to be held virtually.

Several European countries also ended the week with rising coronavirus cases and new restrictions in place in an effort to contain outbreaks without having to revert to major shutdowns.

Amid spiking case counts, Greece on Friday extended a midnight curfew on bars and restaurants, as well as a 50-person cap on public gatherings in areas with increasing coronavirus cases.

Spain on Friday issued a ban on nightclubs, late-night drinking, and smoking and drinking in public after the country recorded 2,935 new cases, including 8,000 since Wednesday. Spain was one of the epicenters of the virus in March and April, and the country went into a strict lockdown to flatten its curve. Since reopening, however, the tourism-reliant country has seen a sharp jump in cases again.

Italy on Friday announced that holidaymakers returning from Spain, Greece, Croatia and Malta would have to be tested for the virus. And Germany expanded its travel warning for Spain, marking the whole country except for the Canary Islands as high-risk and requiring coronavirus tests for all returnees.

Meanwhile, a ban on all “nonessential” travel at U.S. land borders with Canada and Mexico will extend into its sixth month, officials said Friday.

Acting U.S. homeland security secretary Chad Wolf and Canadian Public Safety Minister Bill Blair announced the extension on Twitter.

“We will continue to do what’s necessary to keep our communities safe,” Blair said.

The restrictions were imposed in mid-March and have been extended every month since. The current measures, which were set to expire next week, will remain in place until Sept. 21.

Jacqueline Dupree, Lateshia Beachum, Meryl Kornfield, Miriam Berger and Amanda Coletta contributed to this report.