Pfizer and BioNTech will not receive any money from the government unless their vaccine succeeds in large clinical trials and can be successfully manufactured.
The Pfizer/BioNTech candidate is one of the most advanced of over 150 vaccines being developed against COVID-19.
It is the biggest deal to date under Operation Warp Speed, intended to accelerate the development, manufacturing, and distribution of coronavirus vaccines, therapeutics, and diagnostics.
Trump issued the gloomy warning in his first formal White House virus briefing since the end of April -- and the news sent Asian and European stocks sliding on Wednesday.
The virus has infected more than 14.7 million people and killed over 610,000 of them since emerging in China late last year, with fresh alarm being sounded over its accelerating spread in Africa.