Pfizer said in a statement that there are "no data" to demonstrate that a single dose of its coronavirus vaccine will provide protection from infection after 21 days.

Why it matters: The U.K. announced on Wednesday that it would shift its vaccination strategy "to give as many people in at-risk groups their first dose, rather than providing the required two doses in as short a time as possible." Some provinces in Canada are doing the same.

• Pfizer confirmed in response that although some protection appears to begin as early as 12 days after the first dose, two doses of the vaccine — separated by three weeks — is the only regimen that proved to be 95% effective in Phase 3 trials.
• "Everyone will still receive their second dose and this will be within 12 weeks of their first," the U.K. government added in a statement. "The second dose completes the course and is important for longer term protection."

The big picture: Pfizer's warning comes as many countries, including the U.S., debate how to rapidly deploy the vaccines in the most effective way possible. The U.S. is on pace to fall far below its target of vaccinating 20 million people by the end of 2020, with only 3 million single-doses administered as of Wednesday night.