Epidemiology of measles in the United States, 1997-2001.

Of the 540 measles cases (annual incidence, less than 1/million population) reported during 1997-2001 in the United States, 362 (67%) were associated with international importation: 196 imported cases, 138 cases epidemiologically linked to imported cases, and 28 cases associated with an imported measles virus genotype. The remaining 178 (33%) "unknown-source" cases were analyzed as potential evidence of endemic measles transmission. A total of 83 counties (2.6% of the 3140 US counties) in 27 states reported unknown-source cases; 49 counties reported only 1 unknown-source case, and the maximum reported by any county was 10. Nationally, unknown-source cases were reported in 103 of the 260 weeks. The largest unknown-source outbreak included 13 cases and lasted 5 weeks. The rarity of unknown-source cases, wide gaps in geographic and temporal distribution, and the short duration of the longest unknown-source outbreak indicate that endemic transmission of measles was not sustained in the United States during this period.