The Centers for Disease Control and Prevention and an advisory committee of outside health experts in April began working on a ranking system for what may be an extended rollout in the United States. According to a preliminary plan, any approved vaccines would be offered to vital medical and national security officials first, and then to other essential workers and those considered at high risk — the elderly instead of children, people with underlying conditions instead of the relatively healthy.

Agency officials and the advisers are also considering what has become a contentious option: putting Black and Latino people, who have disproportionately fallen victim to Covid-19, ahead of others in the population.

In private meetings and a recent public session, the issue has provoked calls for racial justice. But some medical experts are not convinced there is a scientific basis for such an option, foresee court challenges or worry that prioritizing minority groups would erode public trust in vaccines at a time when immunization is seen as crucial to ending the pandemic.