Varicella and Zoster, 2009

Minnesota 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 fourth full year of surveillance.

No varicella-related deaths were identified in 2009. Two cases of critical illness due to varicella were reported; both were female. One case was 5 years of age and had a documented history of 1 dose of varicella-containing vaccine. She had an underlying medical condition but was not being treated with immunosuppressive drugs. The case was hospitalized for 1 day with concurrent fever and pneumonia; however, the etiology of the pneumonia was undetermined. The other case was 3 years of age and had not received varicella vaccine. She had no known underlying conditions. The case was hospitalized for 6 days because of complications including cerebellitis and ataxia.

Surveillance includes reporting of outbreaks from schools. An outbreak of varicella in a school is defined as 5 or more cases within a 2-month period in persons <13 years of age, or 3 or more cases within a 2-month period in persons 13 years of age and older. An outbreak is considered over when no new cases occur within 2 months after the last case is no longer contagious. During the 2009-2010 school year, MDH received reports of outbreaks from 20 schools in 16 counties involving 180 students and 2 staff. By comparison MDH received reports of outbreaks from 24 schools in 15 counties involving 261 students and no staff during the 2008-2009 school year. The number of cases per outbreak ranged from 4 to 26 (median, 9) during the 2009-2010 school year and 3 to 39 (median, 8) during the 2008-2009 school year.

Surveillance also includes reporting of individual cases from sentinel schools throughout Minnesota. These data are used to extrapolate to the statewide burden of sporadic disease. For the 2009-2010 school year, 80 sentinel schools were selected; 79 participated. A case of varicella is defined for sentinel school reporting as an illness with acute onset of diffuse (generalized) maculopapulovesicular rash without other apparent cause. During the 2009-2010 school year, MDH received 29 reports of varicella from 18 (23%) sentinel schools. One sentinel school reported a cluster of cases that met the outbreak definition. Five (17%) of 29 reported cases were included in this outbreak. The 24 cases not associated with an outbreak represent sporadic varicella incidence. Based on sentinel school data, an estimated 546 sporadic cases of varicella would have been expected to occur during a school year among the 870,941 total school-aged children (in Minnesota schools with >99 students), representing 0.06% of this population, for an incidence rate of 62.7 per 100,000 population. Estimated grade level-specific annual incidence rates are 107.8 per 100,000 (447 of 414,739) for elementary school students; 26.6 per 100,000 (40 of 148, 935) for middle school students; and 20.8 per 100,000 (59 of 283,870) for high school students.

Beginning in 2007, varicella surveillance included reporting of individual cases from selected sentinel child care sites. In 2009, MDH received no reports of varicella cases from the 120 selected child care sites (including 47 child care centers and 73 sentinel licensed home daycares). Because this program has historically generated very few reports of cases, case-based reporting of varicella in all child care settings was initiated in February 2010.

All suspected or confirmed cases of zoster with disseminated disease or complications other than post-herpetic neuralgia, irrespective of age, are reportable. During 2009, 16 such cases were reported; all were hospitalized. Nine cases were >60 years of age; 4 were 30 to 59 years of age; and 3 were less than 30 years of age, including 1 7-year-old case with aseptic meningitis. Eight of the cases (50%) had underlying conditions or were being treated with immunosuppressive drugs. Seven cases had encephalitis, 3 had meningitis, 3 had severe ocular involvement, 2 had disseminated disease, and 1 had cellulitis. One case with encephalitis subsequently died. Death certificate data were reviewed to identify zoster-related deaths in 2009. Twelve deaths were identified. Cases ranged in age from 39 to 96 years; 1 (8.3%) was <60 years of age.

MDH currently conducts zoster surveillance in all schools. During the 2009-2010 school year, MDH received 94 reports of zoster from schools in 32 counties, representing 0.01% of the total school population of 913,751 for an incidence rate of 10.3 per 100,000. Ages ranged from 6 to 18 years. As opposed to varicella, which is mainly diagnosed by school heath personnel and parents, nearly all (99%) of the 79 zoster cases for whom an interview could be obtained were provider-diagnosed. All cases of zoster in individuals <18 years of age are reportable.

Beginning September 1, 2010, the Minnesota school and child care immunization law will require health care provider verification of varicella disease history. In the past, a parental report was acceptable.

• For up to date information see>> Varicella (Chickenpox)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2009