Key events in development of Pfizer's COVID-19 vaccine

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.

Sept. 8 — With pressure mounting for a vaccine before the U.S. election, CEOs of Pfizer, BioNTech and seven other major drugmakers pledge to stand with the science and not be rushed.

Sept. 12 — Proposal made to expand study enrollment to 44,000 volunteers to include teenagers and people with certain chronic illnesses; regulators later agree.

Oct. 16 — Pfizer says it can't request emergency use of vaccine before the third week of November, when safety information due.

Nov. 8 — Independent board analyzes test results so far and notifies Pfizer.

Nov. 9 — Pfizer announces the vaccine appears to be about 90% effective, based on 94 infections so far in study volunteers.

Late November — Pfizer expects to have more data on effectiveness, along with information on safety and manufacturing quality. Soon after that, Pfizer expects to apply for emergency...