Transmission of Measles Among a Highly Vaccinated School Population - Anchorage, Alaska, 1998

Morbidity and Mortality Weekly Report, Jan 8, 1999

During August 10-November 23, 1998,33 [confirmed.sup.*] measles cases were reported to the Anchorage Department of Health and Human Services and the Alaska Department of Health and Social Services (ADH SS). Of these, 26 cases were confirmed by positive rubeola IgM antibody test, and seven met the clinical case definition. This was the largest outbreak of measles in the United States since 1996(1,2). This report summarizes results of the epidemiologic investigation conducted by ADHSS and underscores the importance of second-dose requirements for measles vaccine.

On August 10, a 4-year-old child (index case) visiting from Japan had rash onset of measles while in Anchorage (Figure 1). The child was hospitalized for 1 day, and measles was diagnosed by positive rubeola 1gM enzyme-linked immunosorbent assay. No measles virus cultures were obtained No cases were reported during the following 3 weeks, when secondary cases would have been expected. On September 5, 26 days after onset of the imported case, a 16-year-old high school student developed measles, confirmed by 1gM testing Subsequently, 15 other students and one teacher at the same high school developed measles during September 14-October 4, 12 cases were laboratory confirmed. In addition, four laboratory-confirmed cases and two clinical cases occurred at six other Anchorage schools, one case-patient attended two schools while infectious (from 7 days before to 4 days after rash onset). Eight other confirmed cases occurred among young adults not associated with schools, and one case occurred in a 2-year-old child.

The 33 case-patients ranged in age from 2 to 28 years (median 16 years) Twenty-nine case-patients had received at least one dose of measles-containing vaccine (MCV) at or after age 12 months, one person with laboratory-confirmed measles had received two appropriately spaced doses of measles-mumps-rubella vaccine (MMR) No serious complications or deaths were reported.

At the high school where the 17 cases occurred, based on school records, only one of 2186 students had not received at least one dose of MCV before the outbreak, 1057 (49%) had received one dose of MCV, and 1112(51%) had received two or more doses Estimated vaccine efficacy for two or more doses of MCV was 100%.

Sequence analysis was conducted on the region coding for the COOH terminus of the nucleoprotein for measles virus cultured from three outbreak cases. All three isolates had identical sequences and were classified as genotype D5 (3). This strain was almost identical to wild measles virus strains circulating in Japan in 1998 and was not related to the strain isolated from an outbreak in Juneau in 1996, the most recent isolate available from Alaska (4).

Before 1996, all students attending public and private schools in Alaska were required to have documentation of a single dose of MCV (or a valid medical or religious exemption). Beginning in September 1996, all students entering kindergarten or first grade were required to have two doses of MCV As a result, school records indicate that virtually all students in kindergarten through third grade as of fall 1998 had received two doses of MMR However, the proportion of students in grades 4-12 that had two doses was unknown.

In response to the outbreak, ADHSS issued an emergency order requiring that all Anchorage schoolchildren have two doses of MCV by November 16, 1998 (Figure 1). Subsequently, the order was expanded to require all students in the state to have two doses of MCV by January 4, 1999 Students were vaccinated by their health-care providers and at special clinics conducted in Anchorage schools By November 17, 98.6% of 49,346. Anchorage School District students had provided documentation of two doses of MCV to their schools.

Reported by B. Chandler, MD, Dept of Health and Human Svcs, Municipality of Anchorage, Alaska State Virology Laboratory; Fairbanks, L. Wood, MPA, E Funk, MD, M. Beller, MD, J. Middaugh, MD, State Epidemiologist, Alaska Dept of Health and Social Svcs Measles Virus Section, Respiratory and Enteric Diseases 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, Div of Applied Public Health Training, Epidemiology Program Office, and an EIS Officer, CDC.

Editorial Note: The occurrence of this outbreak primarily in one school, despite the extremely high one-dose measles vaccine coverage, demonstrates the importance of school requirements for a second dose of MCV. MCV is highly effective, <5% of children who receive one dose fail to develop immunity. However, most children respond to a second dose, and >99% of persons aged [greater then or equal to]12 months receiving two or more doses at least 28 days apart develop immunity.

The Advisory Committee on Immunization Practices and the American Academy of Pediatrics recommend that all students from grades kindergarten through 12 have two doses of MCV by 2001 (5,6). As of the 1998-99 school year, state school requirements for two-dose measles vaccination have covered approximately 53% of U.S. schoolchildren (CDC, unpublished data, 1998). The vigorous response by public health and school officials in Anchorage to this outbreak in accelerating second-dose measles vaccination among schoolchildren may have limited the extent of this outbreak and will help prevent future outbreaks in Alaska schools.