Interstate Measles Transmission from a Ski Resort -- Colorado, 1994

During April 1-May 25, 1994, a chain of measles transmission began in Breckenridge, Colorado, and extended into nine additional states; a total of 247 measles cases were reported, representing 36% of all U.S. measles cases reported to the National Notifiable Diseases Surveillance System (excluding those reported from U.S. territories) through July 2 (week 26). The source of exposure was unknown but is believed to have been an out-of-state tourist who probably visited Breckenridge during March because 1) no measles cases had previously been reported in Colorado during 1994, and 2) the only common exposure appeared to have been at a ski resort visited by many out-of-state travelers. Persons associated with spread of measles from Breckenridge were predominately school- and college-aged. This report summarizes the investigation of this chain of interstate measles transmission.

A total of 15 measles cases with rash onset during April 4-21 occurred in Breckenridge. Persons with measles ranged in age from 16 years to 46 years (median: 27.6 years). All cases met the CDC measles clinical case definition (1); 12 were serologically confirmed. All 15 ill persons either lived in Summit County (Breckenridge) or three neighboring counties (Arapahoe, Chaffee, and Park) or worked in tourism-related services in or near Breckenridge. Twelve of the 15 ill persons are believed to have been exposed to the unidentified source, and three cases resulted from secondary transmission. Two cases occurred among high school students; no further transmission in schools was reported.

Interstate transmission of measles occurred through four out-of-state travelers and a Silver Thorn, Colorado, resident -- all of whom had visited Breckenridge during March 18-25. All five visitors are believed to have been exposed to the unidentified source. Two persons (a 46-year-old Texas resident {rash onset: April 16} and a 29-year-old Missouri resident {rash onset: April 4}) developed measles on return home but have not been linked to additional cases. The other three persons -- an Illinois resident, a Maryland resident, and the Silver Thorn resident -- became sources for further transmission.

Illinois. A 14-year-old unvaccinated female high school student returned home to Jersey County, Illinois; she developed a rash on April 4. The student was identified as the source of an outbreak involving 51 unvaccinated persons (age range: 1-24 years; median: 18 years; last rash onset: June 3) in her community -- which was associated with a Christian Science college in the county. She also was identified as the source of an outbreak involving 156 persons (age range: 4-25 years; median: 15 years; rash onsets: April 17-May 15) at the Christian Science boarding high school she attended in St. Louis County, Missouri. After several unvaccinated persons from other states visited the school during the outbreak, six additional cases occurred. Five persons developed measles on return home (two persons to Maine and one each to California, New York, and Washington); the California patient was the source of exposure for a sibling. No further transmission associated with these six cases is known.

Maryland. A 24-year-old woman returned home to Baltimore County, Maryland; she developed a rash on April 4. The woman was the source of exposure for her 56-year-old father, who had rash onset on April 21.

Michigan. A 25-year-old Silver Thorn man visited his family in Wayne County, Michigan; he developed rash on April 17. The man was identified as the source of an outbreak involving 12 persons (age range: 9 months-37 years; median: 24 years; rash onsets: April 17- May 18) who were exposed at a wedding and a restaurant. One additional case (rash onset: April 16) was reported in a 12-year-old Chicago resident who had visited Wayne County. No further transmission associated with the Michigan or Chicago cases is known.

Reported by: GW Rutherford, III, MD, State Epidemiologist, California State Dept of Health Svcs. RE Hoffman, MD, State Epidemiologist, Colorado Dept of Health. BJ Francis, MD, State Epi-demiologist, Illinois Dept of Public Health. KF Gensheimer, MD, State Epidemiologist, Bur of Health, Maine Dept of Human Svcs. E Israel, MD, State Epidemiologist, Maryland State Dept of Health and Mental Hygiene. KR Wilcox, Jr, MD, State Epidemiologist, Michigan Dept of Public Health. HD Donnell, Jr, MD, State Epidemiologist, Missouri Dept of Health. D Morse, MD, State Epidemiologist, New York State Dept of Health. DM Simpson, MD, State Epidemiologist, Texas Dept of Health. W Lasota, Immunization Program, Washington Dept of Health. National Immunization Program, Office of the Director; Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The sustained interstate measles outbreak described in this report demonstrates the ability of measles virus to spread rapidly and widely among a highly mobile population. The dates of rash onset for the five Breckenridge visitors suggest that they had been exposed to measles during the same period the Breckenridge cases were exposed; therefore, exposure to the common, unidentified source -- not the Breckenridge cases -- probably led to this widespread interstate outbreak. Direct contact of the five visitors with the unidentified source resulted in primary transmission of measles in five other states (222 reported cases), and further contact resulted in secondary transmission in four additional states (six reported cases) before the chain of transmission ended.

Factors that may have contributed to this interstate measles outbreak include 1) the timing of the initial exposure during school spring break; 2) exposure of an unvaccinated student who subsequently returned home to a community and school with many susceptible, unvaccinated persons; and 3) special events at the Missouri boarding school that drew susceptible, unvaccinated visitors from other states.

Although measles spread from Colorado to nine other states, transmission in six states stopped with the index case or after one additional case. In some of these states, spread may have been limited because the sources were adults whose routine activities may not have involved close contact with groups containing susceptible persons. Only two outbreaks (lllinois/Missouri and Michigan) resulted in substantial numbers of reported cases, and both were associated with contact with large groups (e.g., high school and college populations, wedding guests, and restaurant patrons). The extended outbreak in Illinois and Missouri has been the largest measles outbreak in the United States (excluding territories) in 1994 (2).

The primary strategy to prevent measles outbreaks is achieving and sustaining measles vaccination coverage levels of at least 90% for a single dose among all age groups. Efforts are under way to increase measles vaccination coverage among preschool children and implement a recommendation that all school-aged and college-aged persons receive two doses of measles-mumps-rubella vaccine. However, additional strategies may be needed to ensure complete vaccination of adults and to prevent outbreaks in settings where large groups of adults gather (e.g., resorts and restaurants). Large groups that do not routinely accept vaccination will remain potential problems for measles-control programs.

To achieve the Childhood Immunization Initiative's goal of eliminating indigenous measles in the United States by 1996 (3), continued efforts to assure rapid detection of measles cases and implementation of control measures are necessary. To define disease transmission patterns more completely, state and local health departments should rapidly investigate and report all suspected measles cases, obtain laboratory confirmation, determine the vaccination status of each suspected case, and determine the source or chain of disease transmission. Identification of measles cases by transmission category (i.e., international importation, linked to an importation, or indigenously acquired) also will be necessary to track progress toward achieving the 1996 elimination goal.

References

1. CDC. Case definitions for public health surveillance. MMWR 1990;39(no. RR-13):23.

2. CDC. Outbreak of measles among Christian Science students -- Missouri and Illinois, 1994. MMWR 1994;43:463-5.

3. CDC. Reported vaccine-preventable diseases -- United States, 1993, and the Childhood Immunization Initiative. MMWR 1994;43:57-60.

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