Measles--United States, 2000.

In 2000, a provisional total of 86 confirmed measles cases were reported to CDC by state and local health departments, representing a record low and a 14% decrease from the 100 cases reported in each of the previous 2 years. This report describes the epidemiology of measles in the United States during 2000 and documents the continued absence of endemic measles and the continued risk for internationally imported measles cases that might result in indigenous transmission.

IN 2000, A PROVISIONAL TOTAL OF 86 confirmed measles cases were reported to CDC by state and local health departments, representing a record low and a 14% decrease from the 100 cases reported in each of the previous 2 years. 1,2 This report describes the epidemiology of measles in the United States during 2000 and documents the continued absence of endemic measles and the continued risk for internationally imported measles cases that might result in indigenous transmission.
Following state laws and regulations, health-care providers, laboratories, and other health-care personnel report confirmed measles cases to state public health departments; this information is forwarded to CDC. 3 Data on vaccination status, age, complications, setting of transmission, and serologic confirmation of cases also are collected.
Of the 86 reported measles cases, 26 (30%) were internationally imported. * Of the 60 indigenous cases, 18 were import-linked, nine were imported virus, and 33 were of unknown source. Importation-associated cases (i.e., imported, import-linked, and imported virus cases) accounted for 62% of all reported cases.
The proportion of cases classified as "internationally imported" cases has been relatively stable since 1998. Of the 26 imported cases, 14 occurred in United States residents who had traveled abroad and 12 in international visitors. Measles was imported from 10 countries. The largest numbers of imported cases reported were from Japan (seven cases) and Korea and Ethiopia (four each). The states reporting the most imported measles cases were New York (eight cases), California (six), and Hawaii and Vermont (three each). Four counties had more than one imported case in 2000.
On average, imported cases resulted in Ͻ1 import-linked case (range: 0-5). Measles virus was isolated from eight chains of transmission linked to an imported measles case (including three chains of one case). In each chain, the viral genotype sequenced was consistent with the genotype of virus known to be circulating in the source country of the imported case. Virologic evidence of importation was found in five chains of transmission (nine cases) that were not linked epidemiologically to imported cases. Genotype D5 was cultured from two isolated cases and genotypes G2, H1, and H2 were each isolated from one chain of transmission. These genotypes are known to circulate in Japan, China, and Vietnam, respectively. The lack of any consistently repeating genotype indicates that there is no endemic genotype. Therefore, all indigenous cases with genotype information and no epidemiologic link to an imported case were classified as imported virus cases.
During 2000, a total of 20 states reported confirmed measles cases. Three states accounted for 57% of cases: New York (23 [13 from New York City]), California (19), and Nevada (seven). The remaining 17 states each reported from one to three measles cases. Of the 3,140 counties in the United States, 41 (1%) reported a confirmed measles case; seven counties (Ͻ1%) reported more than three cases.
In 2000, 68 (79%) of the 86 reported cases occurred during weeks 1-26, and 18 (21%) occurred during weeks 27-52. The median number of cases per week was one (range: 0-9). During 18 weeks, no cases were reported. During 17 additional weeks, all reported cases were import-associated. During five periods of 4 weeks, all reported cases were import-associated.
In 2000, 10 measles outbreaks (i.e., three or more confirmed cases) occurred in nine states accounting for 48 (56%) of the 86 cases. An epidemiologic link to an imported case was documented in five of the 10 outbreaks.
The largest outbreaks occurred in New York: one in Oswego/Onondaga counties involving nine persons and a second in Kings County involving eight persons. The Oswego/Onondaga outbreak occurred in a high school; the source of infection was unknown. Of the six high school students eligible for vaccination, five had been vaccinated. Each of these students had received a single dose of measles vaccine, which was in compliance with state requirements at that time. The outbreak in Kings County occurred in a religious community in Brooklyn following an imported case from the United Kingdom. Two cases were in infants aged Ͻ12 months. Among the six patients who were vaccine eligible, three were unvaccinated.
One outbreak in 2000 illustrates the difficulty in linking indigenous cases to their imported source. A U.S. resident

FROM THE CENTERS FOR DISEASE CONTROL AND PREVENTION
and Olympic athlete aged 24 years developed prodromal measles symptoms while competing in an athletic event in Utah. The athlete had no known exposure to measles; however, 2 weeks before arriving in Utah, she had participated in an athletic competition in Japan. Following the competition in Utah, the athlete flew to Italy and subsequently developed a rash consistent with measles. The team physician notified CDC of the case from Italy. On return to the United States, the athlete tested IgM positive for measles. Three confirmed measles cases were linked epidemiologically to the athletic event in Utah. No viral strain was obtained from any of the cases.

CDC Editorial Note:
Measles is still endemic in many countries and results in approximately 800,000 deaths per year. 4 However, the reported incidence of measles in the United States has been Ͻ1 case per million for the past 4 years. 1 The high percentage of cases resulting from importations and very limited indigenous spread from these imported cases also has continued over the same period. The consistently small number of unknown source cases suggests that measles is no longer endemic in the United States. However, unknown source cases continue to occur sporadically. Many of these cases, especially isolated cases, might be misclassifications resulting from false-positive laboratory tests. However, even among true measles cases, it is impossible to identify the imported case in every chain of transmission.
The outbreak in Utah demonstrates the difficulty in linking every case to an imported source. CDC was informed of the case only because it occurred in an Olympic athlete. The case was not reported as a U.S. case because rash onset and diagnosis had occurred in Italy. If the team physician had not called from Italy to report this case, the three associated cases in Utah would have been classified as unknown source cases. Because most visits to the United States are of a relatively short duration, many persons shedding measles virus might leave the country before the rash begins and before measles is diagnosed. Many other international visitors who develop measles in the United States might choose to return home before they seek care because they are unfamiliar with the U.S. health-care system or lack valid health insurance in the United States. In both situations, the imported case would not be detected except under special circumstances.
Difficulty in epidemiologically linking every case to an imported source highlights the crucial role of virologic surveillance in monitoring the absence of endemic measles. Collection of viral specimens is an important part of any measles case investigation. Worldwide, during large outbreaks 5,6 or in areas where disease is endemic, 7,8 one measles genotype is usually found. Since 1992 in the United States, no genotypes have been found consistently, and when genotypic data are available, all isolates from imported cases have the genotype found in the country of origin. 5,9 Imported measles cases consistently test the level of population immunity to measles in the United States. The average of less than one importlinked case following an international importation suggests that the level of population immunity is high, probably as a result of successful vaccination efforts in the United States. Firstdose vaccination coverage among preschool children has been Ն90% for the past 4 years. 10  During November 25-January 19, the weekly percentage of patient visits for influenza-like illness (ILI) † reported by U.S. sentinel physicians in 47 states ranged from 1.3% to 2.2%. During week 3, the percentage of visits for ILI was 2.2%, slightly above the national baseline ‡ of 1.9%. During the same week, influenza activity § was reported by state epidemiologists as widespread in Colorado, New York, Utah, and Virginia and regional in 11 states.
During week 3, 7.7% of recorded deaths in the 122 Cities Mortality Reporting System were attributed to pneumonia and influenza (P&I), which is below the epidemic threshold of 8.1% for that week. The percentage of P&I deaths has remained below the epidemic threshold for each week during September 30-January 19. The best protection against influenza is vaccination, and approximately 10 million doses of 2001-02 influenza vaccine remain available. Health-care providers should continue to offer influenza vaccine during February because influenza activity is expected to increase, and unvaccinated persons can benefit from vaccination even after influenza has been detected in their communities. Influenza vaccine is strongly recommended for those at increased risk for serious complications from influenza (e.g., persons aged 6 months-64 years with certain chronic medical conditions and persons aged Ն65 years) and healthcare providers. 2 In addition, household contacts of high-risk persons, healthy persons aged 50-64 years, and any person who wants to reduce their risk for becoming ill with influenza should be vaccinated.
Prompt laboratory diagnosis of influenza can guide clinical decisionmaking and confirm influenza as the cause of respiratory outbreaks in all settings (e.g., nursing homes and hospitals). Immunofluorescence and enzyme immunoassay are available in some laboratories. Commercially available rapid influenza diagnostic tests differ by their ability to detect and distinguish between influenza A and B virus infections, methodologies, processing time, acceptable respiratory specimens, and cost. Some rapid tests are approved for use in a physician's office, and others are considered moderately complex and must be performed at a clinical laboratory. One test detects only influenza A viruses, another test detects and distinguishes between influenza A and B viruses, and three tests detect but do not distinguish between infection with influenza A or B viruses. Respiratory specimens for rapid testing generally should be obtained within 3-4 days of illness onset. The sensitivities of the rapid tests are lower than viral culture of respiratory specimens and a negative result might not exclude influenza virus infection. [3][4][5] When rapid tests are used to detect influenza outbreaks, respiratory specimens also should be obtained and sent for confirmatory viral culture. Information has been published about detection and control of influenza outbreaks in acute-care and long-termcare facilities. [6][7] Antiviral medications can be useful for early treatment of influenza and as an adjunct to influenza vaccination for influenza prevention and control. Influenza antiviral drugs differ in approved ages, recommended dosages, routes of administration, adverse effects, development of antiviral resistance, and cost. When administered within 48 hours of symptom onset, antiviral treatment of influenza can reduce the duration of illness by approximately 1 day in healthy adults. 8 Four prescription antiviral medications (amantadine, rimantadine, oseltamivir, and zanamivir) are approved for treatment of influenza A virus infections. Oseltamivir and zanamivir also are approved for treatment of influenza B virus infections. Antiviral chemoprophylaxis is approximately 70%-90% effective in preventing illness in healthy adults. 8 Amantadine, rimantadine, and oseltamivir are approved for chemoprophylaxis of influenza A virus infections; only oseltamivir is approved for chemoprophylaxis of influenza B virus infections. Physicians should consult the package inserts of the antiviral drugs for information on approved age groups, dosing, and adverse effects.
†Temperature of Ն100.0°F (Ն37.8°C) and either cough or sore throat in the absence of a known cause. ‡The national baseline was calculated as the mean percentage of visits for ILI during noninfluenza weeks plus two standard deviations. Because of wide variability in regional level data, to calculate region-specific baselines is not possible and to apply the national baseline to regional level data is not appropriate. §Levels of activity: (1) no activity, (2) sporadicsporadically occurring ILI or laboratory-confirmed influenza with no outbreaks detected, (3) regionaloutbreaks of ILI or laboratory-confirmed influenza in counties with a combined population of Ͻ50% of the state's population, and (4) widespread-outbreaks of ILI or laboratory-confirmed influenza in counties with a combined population of Ն50% of the state's population.
The expected baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I since 1983. The epidemic threshold is 1.654 standard deviations above the seasonal baseline. Before the 1999-2000 season, a new case definition for a P&I death was introduced. During the summer of 2000, the baseline and epidemic thresholds were adjusted manually to account for these changes in case definition. For the 2001-02 season, sufficient data have been collected using the new case definition to allow projection of the baseline using the regression procedure employed before the 2000-01 season.