Recruiting Black volunteers for vaccine trials during a period of severe mistrust of the federal government and heightened awareness of racial injustice is a formidable task. So far, only about 3 percent of the people who have signed up nationally are Black.

Yet never has their inclusion in a medical study been more urgent. The economic and health impacts of the coronavirus are falling disproportionately hard on communities of color. It is essential, public health experts say, that research reflect diverse participation not only as a matter of social justice and sound practice but, when the vaccine becomes available, to help persuade Black, Latino and Native American people to actually get it. (The participation of Asian people is close to their share of the population.)

People of color face greater exposure to the virus, in part because many work in front line and essential jobs, and have high rates of diabetes, obesity and hypertension, all of which are risk factors for severe Covid-19. But even when those factors are accounted for, people of color still appear to have a higher risk of infection, for reasons researchers cannot yet pinpoint, said Dr. Nelson L. Michael, an infectious-disease expert at the Walter Reed Army Institute of Research.