Varicella and Zoster, 2006

Varicella and zoster surveillance were implemented in Minnesota pursuant to their addition to the Minnesota Rules Governing Communicable Diseases, effective September 13, 2005. The reporting rules require that unusual case incidence, individual critical cases, and deaths related to varicella and zoster be reported. The reporting rules also allow for the use of a sentinel surveillance system to monitor varicella and zoster incidence until that system no longer provides adequate data for epidemiological purposes, at which time case-based surveillance will be implemented. This summary represents the first full year of these surveillance efforts. Over time, these data will be used to monitor trends in varicella and zoster disease in Minnesota, and will be used to extrapolate to the statewide disease burden.

Zero varicella-related deaths were identified in 2006. Two cases of critical varicella illness were reported to MDH. Both had underlying medical conditions. A 14-year-old male with a history of acute lymphoblastic leukemia, treated with immunosuppressant drugs and a bone marrow transplant, developed varicella-related encephalitis. A 1-year-old female with a history of a rare autoimmune disease treated with chemotherapy developed vaccine-associated varicella within 3 weeks of receiving vaccine. Both children recovered.

Varicella surveillance in Minnesota includes reporting of outbreaks from all schools, and reporting of individual cases from selected sentinel schools and childcare centers. Outbreak surveillance was initiated in the fall of the 2005-06 school year, and case-based surveillance at sentinel schools was initiated in January 2006. Forty sentinel schools were selected and participated from January 2006 through the end of the school year, and 80 sentinel schools were selected and participated throughout the 2006-07 school year. Forty childcare centers were selected and participated throughout 2006.An outbreak of varicella in a school is defined as five or more cases within a 2-month period in persons less than 13 years of age, or three or more cases within a 2-month period in persons 13 years of age and older.

An outbreak is considered ended when no new cases occur within 2 months after the last case is no longer contagious. During the 2006-07 school year, MDH received reports of 73 outbreaks from 69 schools (four schools reported two outbreaks) in 30 counties throughout Minnesota involving 1,230 students and zero staff. By comparison, MDH received reports of outbreaks from 76 schools in 33 counties throughout Minnesota involving 1,146 students and two staff during the 2005-06 school year. The number of cases per outbreak ranged from five to 96 (median, 13) during the 2006-07 school year and five to 73 (median, 12) during the 2005-06 school year.

A case of varicella is defined for school and childcare facility reporting as an illness with acute onset of diffuse (generalized) maculopapulovesicular rash without other apparent cause. During the 2006-07 school year, MDH received 192 reports of varicella from 29 (36%) of 80 sentinel schools. Six sentinel schools reported seven clusters of cases that met the outbreak definition (one school reported two outbreaks). One hundred thirty-one (68%) of 192 reported cases were included in these seven outbreaks. Cases per outbreak ranged from five to 57 (median, 8). The 61 cases not associated with an outbreak represent sporadic varicella incidence in Minnesota schools.

Based on sentinel school data, an estimated 951 sporadic cases of varicella would have been expected to occur during a school year among the 883,181 total school-aged children in Minnesota, representing 0.11% of this population, for an incidence rate of 107.7 per 100,000. Estimated grade level-specific annual incidence rates are 166.4 per 100,000 (686 of 412,111) for elementary school students; 75.8 per 100,000 (112 of 148,267) for middle school students; and 51.2 per 100,000 (153 of 299,320) for high school students.

In 2006, MDH received 16 reports of varicella cases from four of 40 sentinel childcare centers. Eleven of 16 cases occurred in the same childcare center. Based on sentinel childcare data, an estimated 1,343 (1.3%) cases of varicella would have been expected to occur during a calendar year among the 107,070 children enrolled in Minnesota childcare centers.

Zoster surveillance in Minnesota currently relies on reporting from health care providers of unusual case incidence, individual critical cases, and deaths, as well as reporting of individual cases in school-age children from all schools, and reporting of individual cases in students and staff from selected childcare centers. The current school reporting system was initiated in November 2005. The current childcare center reporting system was initiated in January 2006. MDH plans to expand zoster surveillance to include all childcare centers, and selected long-term care facilities.

During the 2006-07 school year, MDH received 144 reports of zoster from schools in 37 counties throughout Minnesota. Ages ranged from 5 to 18 years. By comparison, MDH had received 20 reports of zoster in 12 counties throughout Minnesota between November 2005 and the end of the school year. Ages ranged from 5 to 16 years. The increase in reported cases may be attributable to several factors including longer surveillance period, increased awareness of the surveillance system, as well as increased incidence. No cases of zoster were reported by sentinel childcare centers in 2006.

Three varicella-containing vaccines are now approved for use in the United States: varicella vaccine (Varivax), combination measles-mumps-rubella-varicella vaccine (ProQuad), and herpes zoster vaccine (Zostavax). Varivax was licensed in the United States in 1995, and ProQuad was licensed in 2005, for persons 12 months of age and older and 12 months through 12 years of age, respectively. Zostavax was approved in May 2006 for use in persons 60 years of age and older. In 2004, a single dose of varicella vaccine was required for kindergarteners and seventh graders enrolling in Minnesota schools. This requirement will change to two doses of varicella vaccine in 2008. These vaccines will help to decrease the incidence and transmission of varicella in Minnesota schools and the community.

• For up to date information see>> Varicella (Chickenpox)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2006