A new medicine or vaccine takes years to discover and fully develop. Only a miniscule number of small-molecule or biological compounds tested in the laboratory emerge as promising enough to justify investment in a series of highly regulated controlled trials for safety and efficacy.

In Phase I, an experimental compound is tested in a small number of human volunteers to assess safety and tolerability. Phase II clinical trials focus on effectiveness in treating or preventing a disease. Phase III trials test the results of earlier trials in larger populations and gather additional information about effectiveness and safety. (All clinical studies are done by independent investigators.) Registration means that Pfizer has filed an application with a health regulatory authority to obtain marketing approval for a new medicine or vaccine.