FLAGSTAFF, Ariz. (AP) — Few Native American tribes have signed up to participate in clinical trials as coronavirus vaccines are developed. The reasons range from suspicion and distrust tied to unethical practices of the past to the quick nature of the studies, which typically may need several layers of approval from tribes. Researchers say that without participation from Native Americans, tribes won't know which vaccine might best be suited for their citizens. About a handful of tribes have agreed to allow researchers to enroll their citizens in vaccine trials, including in the Southwest and Pacific Northwest. They point to a need to slow the virus among a population that's been disproportionately affected.