The Centers for Disease Control and Prevention — with an American supplier and state health officials — have taken the unprecedented step of devising a rationing plan for the relatively few flu vaccine doses available this season.

Officials announced the new federal distribution program Tuesday. The plan will allocate 10.3 million more doses to states, based on states’ high-risk populations and the number of doses already received.

State health departments can distribute the doses as they see fit. Long-term care residents and workers are expected to receive high priority. Vaccines will be shipped in the next two months as they become available, said Julie Gerberding, the CDC’s director.

Influenza is the fifth-leading cause of death among people age 65 and older, and the elderly accounted for 91% of last year’s flu-related deaths. The vaccine shortage began in early October, when 48 million doses of vaccine intended for the United States were ruled contaminated and impounded.