The U.S. will pay Pfizer and biotech firm BioNTech $1.95 billion to produce and deliver 100 million doses of their Covid-19 vaccine if it proves safe and effective, the companies announced Wednesday.

It was the largest such deal between the government and companies racing to develop a coronavirus vaccine.

Under the agreement, the U.S. can acquire 500 million additional doses, the Department of Health and Human Services said. Germany-based BioNTech and Pfizer are jointly developing four potential vaccines.