Varicella and Zoster, 2007

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 due 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 second 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.

No varicella-related deaths were identified in 2007. Five cases of critical varicella illness were reported. Four had underlying medical conditions and were being treated with immunosuppressive drugs. The other case-patient had no underlying conditions and was not known to be immunosuppressed. Three of the case-patients were male. Race was reported for four case-patients, all of whom were white. Ethnicity was reported for two, both of whom were not Hispanic. Two case-patients had a documented history of one dose of varicella-containing vaccine. Two case-patients had not received varicella-containing vaccine; one was born before 1980 and the other did not specify a reason for not being vaccinated. The other case-patient reported receiving varicella-containing vaccine but this was not verified. Each case-patient was hospitalized for a mean of 5.4 days. Dehydration was the only complication reported in one case-patient. All five case-patients recovered.

Varicella surveillance in Minnesota includes reporting of outbreaks from all schools, and reporting of individual cases from selected sentinel schools and childcare centers. Eighty sentinel schools were selected and participated throughout the 2006-2007 school year and 77 participated in the 2007-2008 school year. One hundred nineteen sentinel childcare centers were selected and participated throughout 2007

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 2007-2008 school year, MDH received reports of outbreaks from 40 schools in 22 counties throughout Minnesota involving 487 students and no staff. By comparison, MDH received reports of outbreaks from 73 schools in 30 counties throughout Minnesota involving 1,230 students and no staff during the 2006-2007 school year. The number of cases per outbreak ranged from five to 37 (median, 9) during the 2007-2008 school year and five to 96 (median, 13) during the 2006-2007 school year.

A case of varicella is defined for sentinel school and childcare facility reporting as an illness with acute onset of diffuse (generalized) maculopapulovesicular rash without other apparent cause. During the 2007-2008 school year, MDH received 67 reports of varicella from 17 (22%) sentinel schools. Four sentinel schools reported clusters of cases that met the outbreak definition. Thirty-nine (58%) of 67 reported cases were included in these four outbreaks. Cases per outbreak ranged from five to 13 (median, 10.5). The 28 cases not associated with an outbreak represent sporadic varicella incidence in Minnesota schools.

Based on sentinel school data, an estimated 678 sporadic cases of varicella would have been expected to occur during a school year among the 876,353 total school-aged children (in Minnesota schools with more than 99 students), representing 0.08% of this population, for an incidence rate of 77.4 per 100,000 population. Estimated grade level-specific annual incidence rates are 141.1 per 100,000 (585 of 414,616) for elementary school students; 52.1 per 100,000 (67 of 128,653) for middle school students; and 8.2 per 100,000 (26 of 315,706) for high school students.

In 2007, MDH received two reports of varicella cases from one (2.1%) of 47 sentinel childcare centers and no reports from 72 sentinel family childcares. Based on sentinel childcare data, an estimated 75 cases of varicella would have been expected to occur during the calendar year among the 95,110 children enrolled in Minnesota childcare centers, representing 0.08% of this population, for an incidence rate of 79.1 per 100,000. No cases of varicella would have been expected to occur during the calendar year among the 142,165 children enrolled in Minnesota family childcares.

MDH currently conducts zoster surveillance in all schools and selected sentinel childcare centers. During the 2007-2008 school year, MDH received 128 reports of zoster from schools in at least 43 counties throughout Minnesota, representing 0.01% of the total school population of 919,176 students for an incidence rate of 13.9 per 100,000. Ages ranged from 5 to 18 years. By comparison, MDH had received 144 reports of zoster in 37 counties throughout Minnesota during the 2006-2007 school year. Ages ranged from 5 to 18 years. As opposed to varicella, which is mainly diagnosed by school heath personnel and parents, most zoster cases (93%) are physician-diagnosed.

In 2007, MDH received one report of zoster from one (2.1%) of 47 sentinel childcare centers and no reports from 72 sentinel family childcares. Based on sentinel childcare data, an estimated 38 cases of zoster would have been expected to occur during the calendar year among the children enrolled in Minnesota childcare centers, representing 0.04% of this population, for an incidence rate of 39.5 per 100,000. No cases of zoster would have been expected to occur during the calendar year among the children enrolled in Minnesota family childcares.

Vaccine supply issues have caused major delays in getting the varicella vaccine to providers, resulting in children unable to receive the second dose of varicella vaccine. It is unclear when the vaccine supply will stabilize. Until it does, the two-dose requirement for kindergarteners and seventh graders enrolling in Minnesota schools has been postponed. However, a single dose of varicella vaccine is still required for students enrolling in these grades, and providers are encouraged to administer the second dose as recommended if varicella vaccine is available.

• For up to date information see>> Varicella (Chickenpox)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2007