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 use authorization in the U.S.

End of year: Pfizer expects to have up to 50 million doses available.