The U.S. will begin exporting Pfizer's COVID-19 vaccine to other countries.

One source familiar with the matter exclusively told Reuters, a shipment of Pfizer's shots going to Mexico this week includes doses made in its U.S. plant.

The source says it's the first time the drugmaker is delivering the vaccines abroad from the U.S.

That's after a Trump-era restriction on exporting Pfizer's shots expired at the end of March.

Pfizer and German partner BioNTech SE have so far been supplying other countries through its main European production plant in Belgium.

Mexico has received 10 million doses of Pfizer's shots so far, and is expected to receive 2 million more this week.

It's unclear how much of that will come from the U.S.

Pfizer said it will be making 25 million shots every week in the U.S. by mid-year.

That would mean successfully honoring a deal to deliver 300 million doses to the country by July.

But as the U.S. swiftly vaccinates its population, many other countries are struggling to acquire vaccine supplies.

They've repeatedly called on Washington to release its surplus in recent weeks.

Pfizer has promised to deliver one billion doses to foreign governments, and said it expects to produce up to 2.5 billion doses of its vaccine this year.

The company said it will use its extra capacity in its U.S. facilities to deliver shots overseas.