A: Millions of people in the U.S. have now received COVID-19 vaccines under what the CDC calls “the most intense safety monitoring in U.S. history” and these vaccines have been shown to be safe and effective.

The shorted timetable doesn’t mean scientists or regulatory agencies are taking shortcuts when evaluating the science, safety and efficacy of COVID-19 vaccines. Usually, the process of moving a vaccine through clinical trials takes years. But several factors led to a significantly shorter-than-usual approval process for the COVID-19 vaccine:

Some clinical trials combined Phase 1 and Phase 2 studies so researchers could quickly evaluate safety and efficacy in a larger population of people at one time. Millions of people were infected worldwide and available to participate in Phase 3 studies.

Distribution began soon after approval because many companies and the U.S. government ramped up manufacturing capabilities before clinical trials were completed. Usually, manufacturing doesn’t begin until all clinical studies are completed, and FDA has reviewed and approved the vaccine.

Learn more about the vaccine development process from Johns Hopkins University.