Coronavirus vaccines are poised for authorization and distribution across the U.S. in the weeks ahead, offering hope that the end of the pandemic is in sight.
The development comes as the coronavirus spreads at a seemingly uncontrollable rate across the country, killing more than 275,000 people and pushing health systems in many states to the breaking point.
Drugmakers and regulators have moved at record speed with vaccine development and trials, creating a sense of optimism but also a host of questions.
Here's what you need to know about the vaccines.
When will the shots be available?
Initial doses could be available in about a week if the Food and Drug Administration (FDA) acts quickly.
The agency is convening a group of expert advisers on Thursday to review the data from Pfizer and its partner BioNTech and could issue an emergency use authorization shortly thereafter. A meeting on the Moderna vaccine is scheduled for Dec. 17.
Vice President Pence said Friday he is “very optimistic” a COVID-19 vaccine may be authorized as early as mid-December.
However, FDA Commissioner Stephen Hahn has been reluctant to put a specific timeline on it, insisting instead that data will drive the decisionmaking.
"I want to set the appropriate expectations. I can tell you that I think we believe it should ... be relatively quick afterward, but there could be issues that come up that we have to address," Hahn said during an interview with ABC News on Tuesday.
"One thing we can't do is promise something that isn't deliverable because of an issue that comes up regarding safety or effectiveness," Hahn said.
The normal vaccine approval process takes months, if not longer. Pfizer filed its application with the agency on Nov. 20.
Hahn has been under pressure to speed up the process from President TrumpDonald TrumpUkraine's president compares UN to 'a retired superhero' Collins to endorse LePage in Maine governor comeback bid Heller won't say if Biden won election MORE, who is eager to claim credit for any COVID-19 vaccine success before he leaves office on Jan. 20.
How many people will be able to get vaccinated this year?
The short answer: not many. Supply is going to be well short of demand.
Operation Warp Speed, the Trump administration’s initiative to speed vaccine development and distribution, plans to send out the first 6.4 million doses of Pfizer's vaccine to states within 24 hours of getting the green light from the FDA. Officials plan to send 12.5 million doses of the Moderna vaccine in the same period.
Additional doses will be sent out in weekly shipments of 5 million to 10 million in what Operation Warp Speed CEO Gen. Gustave Perna said was a "continuous cadence of delivery," but it's still not clear how many doses will be allocated to states.
The first batch will be divided among 69 entities, federal officials said: states, territories, cities and the departments of State, Defense and Veterans Affairs as well as the Bureau of Prisons and the Indian Health Service.
Each vaccine requires two doses, so the first shipment will be enough for only 3.2 million people. The same brand must be used for both shots.
Federal health officials said they expect to have enough doses from Pfizer and Moderna to vaccinate a total of 20 million people by the end of the year, a fraction of the country's 330 million people.
How do the vaccines work?
The vaccines from Moderna and Pfizer use messenger RNA (mRNA) technology, which has never been approved for a human-use vaccine before.
The technology takes advantage of the body's process for making proteins, using a snippet of the virus's genetic code in order to trigger an immune response and build immunity to the virus that causes COVID-19.
Traditional vaccines often use a dead or weakened version of the virus and can carry some small risk of infection in a tiny segment of a vaccinated population. They are also not always completely effective — influenza vaccines can reduce the risk of catching the seasonal flu by between 40 and 60 percent, according to the Centers for Disease Control and Prevention (CDC).
The mRNA technology in both vaccines has been found to be around 95 percent effective in preventing coronavirus infections. It is also extremely safe. The mRNA vaccines carry only the RNA instructions; they do not carry any of the RNA that leads to disease in humans.
Who gets the shots first?
A CDC advisory panel last week recommended that health workers and residents of long-term care facilities should be at the front of the line to receive the first limited doses.
Panel members said they wanted to prioritize health providers to keep the health care system running, and most jurisdictions said they expect to be able to vaccinate every health worker within three weeks of receiving initial doses.
But the final decisions on prioritization are up to individual states, and the initial batches will be allocated based on a state's adult population rather than the proportion of the populations at high risk.
Many states said they will follow the CDC recommendations, but others have their own plans. States also have significant leeway to come up with their own definitions and even create separate sub-prioritization groups.
For example, some states plan to vaccinate front-line health workers and staff of long-term care facilities but not the residents. Some are considering whether to include first responders or teachers in the initial wave of vaccinations.
The exact criteria for who will be first in line will be defined immediately after a vaccine is authorized. But generally, officials expect that after the first wave, the "1b" phase could begin in January or February.
Federal officials don't expect the general population to receive vaccines until the spring or summer.
How much will it cost to get vaccinated?
The Trump administration has vowed to make the coronavirus vaccine free to everyone in the U.S.
The federal government pre-purchased hundreds of millions of doses using tens of billions of dollars in taxpayer money as part of Operation Warp Speed, so the cost has been paid upfront.
Federal rules require most commercial insurers and self-funded employer health plans to provide COVID-19 vaccines with no out-of-pocket costs.
Medicare also recently issued a rule that would completely cover the cost of any authorized COVID-19 vaccine.