ACIP and CDC determined the priority groups, ranked in three tiers,
on the basis of influenza-associated mortality and hospitalization rates (Table).
In the event of an influenza vaccine shortfall, persons in tier 1 should be
vaccinated preferentially, followed by persons in tier 2, then persons in
tier 3. On rare occasions when local vaccine supply is extremely limited,
state and local health officials and vaccination providers should prioritize
persons in group 1A before all other groups. However, in all other vaccine
shortfall situations, persons in groups 1A, 1B, and 1C should be considered
equivalent and should be vaccinated simultaneously. Eligible persons in group
1C and tiers 2 and 3 should be encouraged to receive live, attenuated influenza
vaccine during periods of inactivated influenza vaccine shortfall.