Measles outbreak epidemiology in the United States, 1993-2001.

To evaluate the extent of measles virus circulation and populations at risk in the United States, we reviewed measles outbreaks during 1993-2001. A total of 120 measles outbreaks, constituting 1804 outbreak-related cases, were reported during this period. The maximum outbreak size decreased from 233 cases in 1993-1995 to 119 cases in 1996-1998 and 15 cases in 1999-2001. The maximum outbreak duration decreased from 127 days in 1993-1995 to 65 days in 1999-2001. The majority of outbreaks resulted from documented spread from an internationally imported case (42%) or had a strain of measles virus not endemic in the United States (12%). Outbreaks in which adults were the predominant age group affected accounted for 35% of all outbreaks, compared with 29% of outbreaks predominantly affecting preschool children, 30% predominantly affecting school-aged children and adolescents, and 6% with no predominant age group. The extremely limited size and duration of measles outbreaks indicates very high population immunity to measles and suggests that measles is no longer endemic in the United States.