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of, for example, other age groups and a Phase 4 study drawn from evaluation of those who are inoculated. How would a cohort study work? Inoculation would be provided to a subset of health care workers in some high attack environments, while a control group would receive a placebo. Statistical analysis would compare results from exposure. How would human challenge trials work? Are they morally defensible? We have substantial experience with challenge trials. Volunteers would have to be informed of the risks and probably would be concentrated in age ranges where risks were lowest.