The Walter Reed Army Institute of Research, which is part of the U.S. Department of Defense, began developing the vaccine in March 2016. In September, the Army announced that it had enlisted Sanofi Pasteur, a division of the Paris-based pharmaceutical giant Sanofi, as its research partner. Sanofi was awarded a $43 million grant to conduct a second phase of trials, set to begin in early 2018. If those prove successful, the government has promised Sanofi another $130 million to conduct the third phase of trials.

But that’s not all Sanofi stands to gain if the Zika vaccine works. The Army intends to grant the company an exclusive license to sell the vaccine in the U.S., according to a notice filed in the Federal Register in December.

That arrangement has consumer watchdogs and U.S. lawmakers, including Sen. Bernie Sanders (I-Vt.), raising the alarm. Millions of Americans of reproductive age could need this vaccine to protect against a virus that can spread through sex and cause major birth defects in the children of infected women. And Sanofi ― a company that has previously been accused of jacking up drug prices for American customers ― would be the one setting the cost of this much-anticipated vaccine.

There hasn’t been a decision on this proposed short-term monopoly, nor are the details of the deal public. At this point, there isn’t even a vaccine. But when the Army requested a fair pricing promise from Sanofi, the company . And that has plenty of people worried.