By Peter Loftus and Betsy McKay

Pfizer Inc. and its partner, BioNTech SE, have asked the U.S. Food and Drug Administration to authorize use of their coronavirus vaccine, and an FDA decision could come as soon as this weekend. Moderna Inc. has made a similar request for its shot, and other vaccines could follow. The first rollouts could begin within days.

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. An FDA advisory committee of outside experts is meeting Thursday to review Pfizer's request for authorization of its vaccine, and vote on whether to recommend clearance.

The FDA has scheduled a Dec. 17 advisory committee meeting to consider Moderna's request for authorization. A separate advisory committee to the U.S. Centers for Disease Control and Prevention has voted to recommend that health workers and residents of nursing homes and other long-term care facilities be first in line for the limited number of doses. The same committee will hold additional votes on which groups should be next in line. But governors can make the final call within their states.

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 of Pfizer's vaccine 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.

Of this, about 2.9 million doses will be shipped within 24 hours. A federal official said Wednesday that an additional 2.9 million doses would be held back and shipped about three weeks later for those initial vaccine recipients to get the second of the two-dose regimen. Another 500,000 doses from the initial supply would be held in reserve in case any problems arise, the official said. If Moderna's vaccine is authorized, officials estimate the initial allocation will be about 12.5 million, which may also be sent in separate shipments to accommodate the second injection.

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 30 million people in the U.S. in January and then about 50 million in February, with more in the months following. Globally, Pfizer expects to produce up to 1.3 billion doses in 2021 and Moderna expects up to 1 billion.

Who will get the first doses?

The first doses will likely go to health-care workers and residents of nursing homes and other long-term care facilities, which together number about 24 million. After that, the CDC vaccine advisory committee is considering recommending that essential workers such as teachers, police and food workers get vaccinated, followed by adults with underlying conditions that put the at high risk, and seniors age 65 and older.

The committee hasn't completed its recommendation beyond the first phase, and decisions on which groups get vaccinated when could 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 be vaccinated first, while others are advocating for the most vulnerable -- older Americans -- to be first in line. And some state governors have singled out occupations such as teachers that should be at or near the top of the list. There is a similar debate about whether non-health-care essential workers such as teachers and police should be ahead of adults with high-risk medical conditions and people age 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, according to a study published in October in the American Journal of Preventive Medicine.

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 is 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 expected to be evaluated for effectiveness at preventing infection regardless of symptoms, but those data 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. Federal and 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?

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12-10-20 1124ET