The Trump Administration will pay Pfizer nearly US$2 billion ($3b) for a December delivery of 100 million doses of a Covid-19 vaccine the pharmaceutical company is developing, Health and Human Services Secretary Alex Azar announced.
The US could buy another 500 million doses under the agreement, Azar said.
"Now those would, of course, have to be safe and effective" and approved by the Food and Drug Administration, Azar said during an appearance on Fox News.
Pfizer Inc. and BioNTech SE announced separately that the agreement is with HHS and the Defence Department for a vaccine candidate the companies are developing jointly. It is the latest in a series of similar agreements with other vaccine companies.
The agreement is part of US President Donald Trump's Operation Warp Speed vaccine programme, under which multiple Covid-19 vaccines are being developed simultaneously. The programme aims to deliver 300 million doses of a safe and effective Covid-19 vaccine by January 2021.
Under the initiative, the government will speed development and buy vaccines — before they are deemed safe and effective — so that the medication can be in hand and quickly distributed once the FDA approves or authorises its emergency use after clinical trials.
Pfizer and BioNTech said Americans will receive the vaccine for free.
Azar said the contract brings to five the number of potential coronavirus vaccines that are under development with US funding. Nearly two dozen are in various stages of human testing around the world, with several entering final test to prove if they really work.
Other countries are also scrambling to get a vaccine.
Britain announced on Tuesday that it had secured access to another 90 million experimental Covid-19 vaccines made by Pfizer and others, a move some campaigners warned could worsen a global scramble by rich countries to hoard the world's limited supply of Covid-19 vaccines.
China, where the new coronavirus originated, also has several vaccine candidates entering final testing.
The FDA has told manufacturers it expects any vaccine to be at least 50 per cent effective to qualify. But at a congressional hearing, Congressman Frank Pallone, D, said he was worried US President Donald Trump could push the agency into prematurely clearing a vaccine.
"My fear is that FDA will be forced by the Trump Administration to approve a vaccine that lacks effectiveness," Pallone said.
Executives from five leading vaccine companies testified that they will take no shortcuts in their testing of the shots, so that people can be confident in the results. In addition, it won't be just the FDA rendering an opinion - each vaccine will likely be judged nearly simultaneously by regulatory authorities in Britain and Europe.