Key events in the development of a COVID-19 vaccine by partners Pfizer Inc. and Germany's BioNTech:
March 17 — Pfizer and BioNTech announce plans to jointly develop a COVID-19 vaccine using BioNTech's technology.
April 29 — Testing of four vaccine candidates begins with volunteers in Germany, one of six countries in the testing plan.
May 5 — Testing expands to the United States.
July 1 — Preliminary data shows one of four candidates appears to stimulate the immune system, is well tolerated.
July 22 — U.S. government agrees to buy 100 million vaccine doses for $1.95 billion, with an option for 500 million more. Several other countries also sign agreements for vaccines.
July 27 — Late-stage tests begin for 30,000 volunteers; companies could seek U.S. regulatory approval as early as October if all goes well.
Aug. 20 — The partners announce promising early data from testing of their lead vaccine candidate.