Pfizer and BioNTech have halved their original estimates for how many coronavirus vaccines would be shipped globally by the end of this year, citing supply-chain issues, the Wall Street Journal first reported.

Why it matters: The U.K. government has ordered 40 million doses of Pfizer-BioNTech's vaccine — enough to inoculate some 20 million people. The companies now expect to ship 50 million vaccines by the end of 2020, per WSJ.

• "Based on current projections we expect to produce globally up to 50 million vaccine doses in 2020 and up to 1.3 billion doses in 2021," Pfizer spokesperson Kim Bencker told Axios.

What they're saying: “[S]caling up a vaccine at this pace is unprecedented, and we have made significant progress as we have moved forwards in the unknown,” Bencker said. “And it’s important to highlight that the outcome of the clinical trial was somewhat later than the initial projection requiring us to focus additional efforts on clinical trial production."

• Pfizer did not specify to the Journal what shortfalls over ingredients and raw materials — which were sourced from Europe and the U.S. — took place as production ramped up.

The big picture: The global race to develop a COVID vaccine, which has seen hundreds of candidates, has been one of the fastest in history. It usually takes more than 10 years for a vaccine to reach regulatory approval after its discovery.

• Pfizer's vaccine has been shown to be more than 90% effective.

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