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Measles --- United States, 1999
State and local health departments reported a provisional total of 100 confirmed measles cases to CDC in 1999. This total equals the record low number of cases reported in 1998 (1). Since 1997, measles incidence in the United States has remained <0.5 cases per 1,000,000 population. This report describes the epidemiology of measles during 1999, which indicates that measles is not endemic in the United States.
Of the 100 cases reported during 1999, 33 were imported, and 67 were indigenous.* Of the 67 indigenous cases, 33 were import-linked and 34 were unknown-source cases. Although some import-linked cases had supporting virologic evidence, no reports relied solely on virologic evidence for classification.
Imported cases accounted for 33% of all measles cases reported in 1999, continuing a trend since 1992 of an increased proportion of imported cases (Figure 1). Imported measles cases occurred among 14 international visitors and 19 U.S. residents exposed to measles while traveling abroad.
Imported cases, by World Health Organization (WHO) region, included 10 from the Western Pacific region, six each from the Eastern Mediterranean, European, and South East Asia regions, two from the American region, and one from the African region. The source region of two cases was unknown.
Persons with imported cases transmitted measles virus to 33 persons with import-linked measles cases. The average number of import-linked cases spread from each imported case was one (range: 0--14). Virologic evidence of importation was found in seven chains of transmission, including seven imported cases and 26 import-linked cases. In each chain, the viral genotype identified was consistent with the genotype of virus known to be circulating in the source country of the imported case. In the chains of transmission associated with imported cases from England, Italy, and Sweden, a new measles virus genotype was isolated. This new genotype is proposed by the WHO measles strain bank to be labeled D7.
In 1999, the proportion of all cases classified as unknown source cases was 34%; this proportion has been decreasing since 1995 (Figure 2). Of the 34 unknown-source cases, 10 were isolated cases with no epidemiologic link to any other measles case. The remaining 24 cases occurred in four outbreaks.
Geographic and Temporal Patterns of Distribution
During 1999, 31 states and the District of Columbia reported no confirmed measles cases. Ten states accounted for 86% of cases. Unknown source cases were reported from nine states. During 33 weeks, all reported measles cases were importation-associated (no unknown source cases were reported), including cases reported during a continuous period of 12 weeks (weeks 19--30).
Of the 3140 counties in the United States, 16 (0.5%) reported measles cases of unknown source. In 10 of these counties, unknown source cases occurred during 1-week periods. Five counties reported unknown source cases for periods between 2 and 4 weeks, and one county reported unknown source cases during 11 noncontinuous weeks.
Age and Vaccination Status
During 1999, persons aged >20 years accounted for 32% of reported measles cases. Elementary school-aged children and adolescents (aged 5--19 years) accounted for 26% of cases, followed by preschool children (aged 1--4 years) with 24% of cases, and infants (aged <1 year) with 18% of cases.
Among the 100 persons with measles, 16 had been vaccinated with one or more doses of measles-containing vaccine. Measles vaccination rates were 0% among infants, 17% among preschool-aged children, 19% among school-aged children including adolescents, and 22% among persons aged>20 years. Among U.S. residents with measles, 15 (17%) of 86 were vaccinated, compared with one (7%) of 14 among inter-national visitors.
Eleven measles outbreaks (a cluster of three or more cases) with a median of four cases per outbreak were reported in 1999 and accounted for 63% of all cases reported during 1999. An epidemiologic link to an imported measles case was documented in seven of the outbreaks.
The largest outbreak (15 cases) during 1999 occurred in Bedford, Virginia. The index case-patient was an adult who had traveled through Europe, Africa, and the Middle East. Fourteen cases occurred in three generations of spread. Settings of transmission included the household and church of the index case-patient and health-care settings.
Reported by: State and local health depts. Measles Virus Section, Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Measles Elimination Activity, Child Vaccine Preventable Diseases Br, Epidemiology and Surveillance Div, National Immunization Program; and an EIS officer, CDC.
The findings in this report document a continuing trend of record low numbers of measles cases and a high percentage of imported cases, suggesting that measles is not endemic in the United States. In 1999, as in the previous 2 years, few measles cases of unknown source were reported and these cases did not cluster temporally or geographically in patterns that would suggest a chain of endemic transmission. Virologic data indicated that only imported virus strains were transmitted in the United States in 1999.
During March 2000, CDC convened a consultation of measles experts§ to evaluate data on the elimination of endemic measles from the United States. The data indicated that, during 1997--1999, measles incidence has remained low (<0.5 cases per 1,000,000 population) and that most states and 99% of counties reported no measles cases. In addition, measles surveillance was sensitive enough to consistently detect imported cases, isolated cases, and small outbreaks. Evidence of high population immunity included coverage of >90% with the first dose of measles vaccine in children aged 19--35 months since 1996 (2) and 98% coverage among children entering school (3). In 48 states and the District of Columbia, a second dose of measles vaccine is required for school entry (4). A national serosurvey indicated that 93% of persons aged >6 years have antibody to measles (5).
On the basis of these findings, the experts concluded that measles is no longer endemic in the United States. However, because endemic measles could be reestablished if vaccination coverage declines, efforts should continue to ensure that coverage remains high and that surveillance is strong. In addition, because of the continued threat of imported measles, the experts encouraged strengthened support for global measles control and eradication of measles.
* Imported=cases among persons who were infected outside the United States; indigenous=cases in persons infected in the United States.
Indigenous cases are subclassified into three groups: import-linked=cases epidemiologically linked to an imported case (virologic evidence of importation is not required for this classification); imported virus=cases that cannot be linked epidemiologically to an imported case, but for which imported virus has been isolated from the case or from an epidemologically linked case; and unknown source=includes all other cases acquired in the United States for which no epidemiologic link or virologic evidence is found to indicate importation.
§ Experts included representatives from the American Academy of Family Physicians, the American Academy of Pediatrics, the Advisory Committee on Immunization Practices, the Council of State and Territorial Epidemiologists, and the National Vaccine Advisory Committee.
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