By Peter Loftus & Betsy McKay

Pfizer Inc. and its partner, BioNTech SE, have asked the U.S. Food and Drug Administration to authorize use of their coronavirus vaccines. Moderna Inc. is getting close to making a similar request for its shot, and other vaccines could follow. The first rollouts could begin as soon as December.

Here is what we know and don’t know about how, and when, the vaccine will get to you.

How will the Covid-19 vaccines be approved, and who decides who will get them?

The FDA will determine whether to authorize Covid-19 vaccines for use. Decisions might come in December for the Pfizer and Moderna vaccines. An advisory committee to the U.S. Centers for Disease Control and Prevention is expected to recommend who should be first in line for the initial limited number of doses. But governors can make the final call within their states, Health and Human Services Secretary Alex Azar said Tuesday.

How will the vaccines be distributed?

The federal government has a contract with McKesson Corp. to be a centralized distributor of Covid-19 vaccines, with the exception of Pfizer’s. Pfizer has set up its own distribution network. Federal health officials say initial doses would be shipped within 24 hours of any FDA authorization, and immunizations could begin within about 48 hours. The federal government also has partnerships with national pharmacy chains CVS and Walgreens to vaccinate residents and staff at long-term care facilities.

Some experts say it could take more than 48 hours for dosing to begin, as hospital workers and others get used to procedures for opening specialized, temperature-controlled boxes of vaccine vials and learn the risks and benefits of the shots.

“Many providers are going to need a few days to get it up and running, if not a week,” said Claire Hannan, executive director of the Association of Immunization Managers, whose members run state, territorial and local vaccination programs.

What logistics are in place to deliver the vaccines?

McKesson, the centralized distributor for vaccines other than Pfizer’s, also will receive and package kits of medical supplies needed to administer the Covid-19 vaccine, such as needles and syringes and alcohol prep pads. It will send the kits and vials of the vaccine out to pharmacies, doctors’ offices and other facilities, at a minimum of 100 doses per order, based on order information supplied by the CDC.

Pfizer plans to use its own distribution centers and ship its vaccine in specially designed reusable containers that can keep thousands of doses at the ultracold temperatures required for it.

How many doses will be available at first?

The initial expected supply immediately after authorization is about 6.4 million doses, according to Gen. Gustave Perna, chief operating officer of the U.S. government’s Operation Warp Speed initiative. Including that initial supply, federal officials have estimated there would be enough doses to vaccinate 20 million Americans in December.

How many doses will be available next year?

Federal officials have estimated there could be enough to vaccinate about 25 million to 30 million people a month starting in January.

Who will get the first doses?

The first doses are likely to go to health-care professionals, workers in essential jobs, nursing-home residents, older Americans and people with underlying conditions that put them at high risk. Exactly which of those groups goes first would depend in part on the particular vaccine and what its data show about effectiveness among different age groups or health conditions.

Is there any debate about who should get vaccinated first?

Yes. Some health officials and experts believe health-care workers should receive the first doses, while others are advocating for the most vulnerable—older Americans—to be first in line.

The vaccine panel advising the CDC is considering recommending that about 3 million elderly people in congregate settings like nursing homes be part of the first phase of immunization along with about 21 million health workers. The committee also has proposed putting essential workers such as teachers and police ahead of adults with high-risk medical conditions and people ages 65 and over who aren’t in congregate settings.

When can the general public expect to have access?

Mr. Azar said he expects there to be enough vaccine doses starting in the second quarter of 2021 so that anyone who wants a vaccine can get it. Other federal health officials have said in the spring or summer. The timeline could change if manufacturing doesn’t go as planned.

How will vaccine doses be allocated to U.S. states?

For the initial supplies, the federal government plans to allocate doses to states proportionally based on the size of their adult populations. It is unclear how long the federal government would stick with population-based proportions and how it would allocate supplies later.

How do states decide to distribute doses?

State, territorial and some local immunization programs, working with the CDC, have drawn up plans to distribute doses within their jurisdictions and to conduct vaccination campaigns. These plans include identifying facilities where vaccination campaigns can be conducted, enrolling them and ensuring the necessary equipment is in place to conduct them. States also have estimated their populations of high-priority groups like health-care workers.

Does the vaccine work the same way in all population groups?

Pfizer and Moderna haven’t yet provided full breakdowns of vaccine efficacy by age and race or ethnicity, but the companies have said efficacy was consistent across these groups.

Does everyone get the same dose regardless of age or other demographic?

Yes.

How many people need to get vaccinated to stop the pandemic in the U.S.?

Moncef Slaoui, chief adviser to Operation Warp Speed, has said if 70% of the population were immunized, that level would achieve herd immunity, based on the approximately 95% effectiveness of both the Pfizer and Moderna vaccines.

A vaccine would need to be at least 80% effective, with about 75% of a population receiving it, to extinguish an epidemic without any other public-health measures.

Reaching those levels of immunization would require educating millions of Americans about the safety and effectiveness of vaccines and confronting a strong antivaccine movement, said Peter Hotez, a vaccine scientist at the Baylor College of Medicine and an author of the paper. Those are steps the government hasn’t taken yet, he said. “To use a vaccine to eliminate this virus—it’s a really high bar,” he said.

One open question is how effective the vaccines are at preventing people from transmitting the virus to others, Dr. Hotez said. Both vaccines were tested primarily for their effectiveness at preventing people from becoming ill. They are also being evaluated for effectiveness at preventing infection regardless of symptoms, but data on those matters haven’t been released yet.

What is herd immunity?

Epidemiologists estimate that between 60% and 70% of a population needs to develop an immune response to the virus to reach “herd immunity,” a state in which enough people have either been infected or vaccinated to stop transmission of the virus. Some epidemiologists say herd immunity to Covid-19 might be achieved at a lower threshold of 50%.

When the vaccines are widely available, how will I get the shot?

Federal officials say they want to make getting a Covid-19 vaccine as easy as going to a pharmacy to get a flu shot. The government has formed partnerships with about 60% of U.S. pharmacies to administer Covid-19 vaccines to the broader population after high-priority groups are vaccinated. Manufacturers would ship doses to distributors to get them to hospitals, pharmacies, nursing homes and other administration sites, as determined by state and federal plans. Pfizer’s vaccine requires ultracold shipping and storage, while Moderna’s can be shipped at higher—though still freezing—temperatures. After thawing, doses can be kept in refrigerators for certain periods.

How many doses will I need?

Vaccines from Pfizer, Moderna and AstraZeneca PLC are given in two doses, three or four weeks apart. State officials are planning to issue reminders to people to come back for their second doses. A Johnson & Johnson vaccine is being tested as a single dose, but the company hasn’t yet reported how well that works.

How much does it cost? Will insurance cover it?

Both the Trump administration and President-elect Joe Biden have said the vaccine would be free of charge to all Americans, with administration fees billed to private or government insurance plans or to a special government relief fund for the uninsured.

Does it have to be a needle?

The vaccines closest to authorization are given as injections. Merck & Co. is exploring an oral formulation of a Covid-19 vaccine, but it isn’t expected to be available in the near term.