United Airlines has begun flying Pfizer and BioNTech’s coronavirus vaccine within the U.S. on passenger flights.

A United spokeswoman told The Hill that doses were on passenger flights after the Food and Drug Administration granted emergency approval for the vaccine last Friday.

The airline said it could not give specific dates or routes due to customer agreements and security risks. The move was first reported by CNBC.

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“The scientists who developed these vaccines and the frontline workers who are administering them are the real heroes,” the airline said in its statement. “We’re proud to play a small part in helping to get this precious cargo to the people who need it most.”

United flew five cargo-only flights delivering the vaccine from Brussels to Chicago earlier this month. The Federal Aviation Administration is reportedly allowing United to carry five times the acceptable limit of dry ice to transport the vaccine.

Pfizer’s vaccine needs to be stored at ultra-cold temperatures to maintain its efficacy.

“We’ll continue to operate cargo-only flights to transport the vaccine and are also now making use of passenger flights in coordination with our shipping and logistics partners to distribute even more treatments to destinations within our network,” the airline said.

Vaccinations using Pfizer’s vaccine began across the country this week as the U.S. grapples with the raging pandemic.

Meanwhile, an independent panel of experts is expected to meet this week to vote on Moderna’s coronavirus vaccine, making an emergency approval likely as early as Friday. The FDA released documents on Tuesday confirming that it is 94 percent effective against severe disease.