A word about how clinical trials work. Candidate vaccines and potential drugs spend years progressing through lab studies, then animal studies, and then tests in humans. The current trials are able to move so fast because the National Institutes of Health is permitting vaccine-makers to skip or delay animal testing. Instead, vaccine-makers are taking their formulas straight into human testing, in three steps called phases.
Phase 1, usually done in fewer than a hundred volunteers, tests for the vaccine candidate’s safety and also tries to determine the right dose. Phase 2 expands the pool to several hundred people, continues to look for safety issues and side effects, and starts to examine whether the vaccine evokes an immune response. Phase 3 uses thousands of people—for the current vaccine trials, an estimated 20,000 to 30,000—who are divided between those getting the vaccine candidate and others getting a placebo, to determine whether the