Interstate measles transmission from a ski resort - Colorado, 1994

Morbidity and Mortality Weekly Report, Sept 2, 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 meales 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 tourismrelated 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.

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.