So that’s how the vaccine trials were designed: Each would try to demonstrate at least 50 percent efficacy with respect to
symptomatic disease as its “primary outcome.”
The trials could have used severe disease, hospitalization, or death as primary outcomes, but that would have slowed things down. These events are far more infrequent—there could have been
200 infections for each COVID-19 death in the U.S.—and that means it would have taken more time, and larger numbers of trial participants, to generate enough data to be sure of a 50 percent efficacy. Developers did include “severe COVID-19” as a secondary outcome—that is, one that would be measured and analyzed, but for which the trial might not have been designed to provide a definitive answer.
Efficacy against hospitalization and against death, however, were not included as secondary outcomes for every trial.