No decision has been made, but the consensus among many experts in the U.S. and globally is that health care workers should be first, said Sema Sgaier of the Surgo Foundation, a nonprofit group working on vaccine allocation issues.

An expert panel advising the U.S. Centers for Disease Control and Prevention is also considering giving high priority to workers in essential industries, people with certain medical conditions and people age 65 and older.

Once a vaccine gets a green light from the Food and Drug Administration, the panel will look at clinical trial data on side effects and how people of various ages, ethnicities and health statuses responded. That will determine the panel's recommendations to the CDC on how to prioritize shots.

State officials are expected to follow the CDC's guidance as they distribute the first vaccines.

Vaccine supplies will be limited at first. There won’t be enough to protect everyone, yet getting the shots to the right people could change the course of the pandemic.

Many other questions about distribution remain unanswered, Sgaier noted, such as whether to distribute shots equally across the country, or to focus on areas that are hot spots.