The United States approved the nation's first Covid-19 vaccine on Friday, marking what could be the beginning of the end of an outbreak that has killed more than 300,000 Americans.
Injections for health workers and nursing home residents were expected to begin in the coming days after the Food and Drug Administration authorised emergency use of what research suggested is a strongly protective vaccine made by Pfizer and its German partner BioNTech.
"The first vaccine will be administered in less than 24 hours," US President Donald Trump said in a televised address released on Twitter.
"Through our partnership with FedEx and UPS, we have already begun shipping the vaccine to every state and zip code in the country," he said, adding that governors would decide who would receive the shots first in their states.
"We want our senior citizens, health care workers and first responders to be first in line," said Mr Trump. "This will quickly and dramatically reduce deaths and hospitalisations."
Initial doses were rationed as the US joined Britain and other countries in scrambling to vaccinate as many people as possible ahead of winter. It will take months of work to tamp down the coronavirus that has surged to catastrophic levels in recent weeks and has claimed 1.6 million lives around the world.
While the FDA decision came after a public review of data from an ongoing study, it was dogged by intense political pressure from the Trump administration, which accused the agency of being too slow and threatened to remove chief Stephen Hahn if a ruling was not made on Friday.
The move sets off what will be the largest vaccination campaign in US history - but it also has global ramifications because it is a pathfinder for many other countries facing the same decision.
It offers the ability “in this situation where the pandemic is out of control, to bring hope to the people”, Dr Ugur Sahin, chief executive of BioNTech, told The Associated Press.
The world needs multiple vaccines to meet demand, and the Pfizer-BioNTech inoculation is the first based on rigorous scientific testing to emerge from that race - a record-setting achievement that took years off the usual process.
“I don’t think you would have found a scientist on this planet that would have predicted this 11 months ago,” said Dr Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia who advises the FDA.
The US is considering a second vaccine, made by Moderna, that could be put to use in another week. In January, Johnson & Johnson expected to learn if its vaccine is working in final testing.
Europe is set to make its own decision on the Pfizer-BioNTech and Moderna shots this month, an important step as some other candidates that multiple countries were awaiting have hit roadblocks. On Friday, Sanofi and GSK announced a months-long delay after early tests showed their vaccine did not work well enough in older adults.
China and Russia did not wait for final-stage tests before beginning vaccinations with some homegrown shots.
About three million doses of the Pfizer-BioNTech vaccine are expected in the first shipments around the US, according to officials with Operation Warp Speed, the Trump administration’s vaccine development programme. A similar amount is to be held in reserve for those recipients’ second dose.
A Centres for Disease Control and Prevention advisory committee was poised to recommend who’s first in line.
Expected to follow health workers and nursing homes are other essential workers, older adults and people at high risk because of other health problems. US authorities do not expect enough for the general population before spring, and that is assuming there are no manufacturing glitches.
“We would need at least until March, April, to have an impact on the pandemic,” BioNTech’s Mr Sahin said. But he predicted that protecting the most vulnerable could start putting a dent in hospitals stays and deaths sooner.
In a still unfinished study of nearly 44,000 people, the FDA found the vaccine was safe and more than 90 per cent effective across recipients of different ages, including older adults, races and those with health problems that put them at high risk from the coronavirus.
Emergency use means the vaccine still is experimental.