Pfizer's vaccine authorization means 2.9 million high-risk people will get the shot within days - the first step in a 9-month path to normalcy

• The FDA has authorized the first coronavirus vaccine in the US, a two-dose regimen from Pfizer and BioNTech.
• Some frontline healthcare workers and residents of long-term care facilities could receive the shots in as little as 24 hours, likely on Monday.
• The US could immunize 20 million people by the end of 2020, but social distancing and mask wearing will need to continue well into 2021.

It's the news Americans have been waiting on: The first coronavirus vaccine has been authorized by the Food and Drug Administration, clearing it for widespread emergency use in the US.

The two-dose regimen was developed by US pharmaceutical company Pfizer and German biotechnology company BioNTech. The vaccine was 95% effective in preventing COVID-19 in clinical trials.

The third phase of the trial involved more than 43,000 volunteers across six countries and 16 US states. Just eight COVID-19 cases were recorded in the group that got the vaccine, compared to 162 cases in the placebo group.

An independent FDA advisory committee met to discuss that data on Thursday. The committee determined the vaccine was safe to administer to the public, since most immunized volunteers just reported temporary side effects like fatigue, headache, and pain at the injection site.

"Today's action follows an open and transparent review process that included input from independent scientific and public health experts and a thorough evaluation by the agency's career scientists to ensure this vaccine met FDA's rigorous, scientific standards for safety, effectiveness, and manufacturing quality needed to support emergency use authorization," FDA Commissioner Stephen Hahn said in a statement.

The FDA's authorization, he added, "is a significant milestone in battling this devastating pandemic that has affected so many families in the United States and around the world."

The Centers for Disease Control and Prevention has recommended that the nation's 21 million healthcare workers and 3 million residents of long-term care facilities - including assisted living and nursing homes - get priority access to the shots.

With the FDA's authorization, Pfizer can distribute shots using its own delivery system, which is centered in Kalamazoo, Michigan, and Pleasant Prairie, Wisconsin. Pfizer has 6.4 million vaccine doses available, but the company's first shipments will only contain 2.9 million doses. That's because the 2.9 million people who get those early shots will need second doses after three weeks. Another 500,000 doses are being held back for emergency situations.

Some individuals could receive their first shot within 24 to 48 hours. Then they'll need a second shot 21 days later (a single dose is only 52% effective at preventing COVID-19).

Hospitals must decide, however, how to allocate the shots among staff. Some many use a lottery system, though many will likely select employees with the highest risk of exposure to COVID-19, such as emergency physicians.

To help nursing homes administer vaccines to residents, the federal government has contracted with CVS and Walgreens.

Since states were required to come up with their own distribution plans, some have elected to prioritize certain vulnerable groups over others. Mississippi will give its first doses to hospital workers and first responders, including police officers and firefighters. Utah is also planning to deliver doses to frontline workers before residents and staff at long-term care facilities.

New York, on the other hand, will give nursing home residents first access to its 170,000 doses, followed by nursing home staff and high-risk hospital workers. And Kentucky will divide up its first batch, giving 12,600 doses to hospitals and 25,400 doses to long-term care residents and staff.

Vaccines for priority groups will continue into 2021

Not all 24 million Americans in the first priority group will get vaccinated before January.

Federal officials estimate that the US could distribute 40 million doses by the end of 2020 - enough to immunize 20 million people. That total includes doses from both Pfizer and Moderna.

The FDA advisory committee will evaluate the vaccine from Moderna on December 17. Assuming its shot gets authorized quickly, the company expects to distribute 20 million doses before the end of the year. Trial data has shown Moderna's shot to be 94.5% effective at preventing COVID-19.

Other priority groups, including people over 65 and those with preexisting health conditions, are likely to get vaccinated at the start of 2021.

Moncef Slaoui, who is spearheading Operation Warp Speed, recently estimated that the US could immunize 100 million people by the end of February. That would require another 60 million doses to be distributed in January, followed by 100 million in February.

By April, vaccines could start being distributed to young, healthy Americans. Moderna CEO Stéphane Bancel recently told Business Insider that any American who wants a vaccine should have access to one by Memorial Day.

Slaoui recently told the Washington Post that most Americans could be immunized by mid-year 2021.

Americans could breathe a sigh of relief in fall 2021

Vaccine authorization doesn't bring an automatic end to the pandemic.

Researchers still don't know whether coronavirus vaccines are safe for young children, pregnant women, or immunocompromised individuals. It's also not known yet whether the vaccines prevent people from getting infected or spreading the virus. Cases were only tested and counted based on symptoms, so it's possible some trial participants got asymptomatic infections. If the shots only prevent symptoms, it could take longer for transmission to slow in the US.

Public-health experts say most of 2021 will likely involve some degree of mask wearing and social distancing, particularly in areas where transmission is high.

"We're still going to have to change our habits a little bit, realistically, until we get to next fall," Megan Ranney, an emergency-medicine physician at Brown University, told Business Insider. "Most of us in public health are looking at kind of late summer, early fall as being when things really get back to normal."

But over the spring, she added, Americans could "start to see a light at the end of the tunnel."

By the end of summer, outdoor concerts could resume and corporations may start to fully reopen office buildings, she said. Universities could also resume in-person classes in the fall semester.

"It will not be until the fall that we'll be able to take a true sigh of relief," Ranney said.

Still, she added, "we are probably going to wear masks on airplanes or trains or buses for a long time to come."