Varicella and Zoster, 2008

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 third 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 2008. Five cases of critical varicella illness were reported. Three had underlying medical conditions but were not being treated with immunosuppressive drugs. The other case-patients had no or unknown underlying conditions and were not known to be immunosuppressed. Three of the case-patients were female. One case-patient had a documented history of one dose of varicella-containing vaccine. Three case-patients had not received varicella-containing vaccine; one was underage for vaccination and two were not vaccinated due to their immunosuppressive conditions. The other case-patient’s vaccination history was unknown. Each case-patient was hospitalized for a mean of 4.4 days. Complications were reported in four case-patients and included varicella arteriopathy, encephalitis, probable hepatitis secondary to varicella, and difficulty swallowing. All five case-patients recovered.

Varicella surveillance 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, 77 participated in the 2007-2008 school year and 80 participated in the 2008-2009 school year. One hundred nineteen sentinel childcare centers were selected and participated throughout 2007, and 120 participated in 2008.

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 2008-2009 school year, we received reports of outbreaks from 24 schools in 15 counties involving 261 students and no staff. By comparison, MDH received reports of outbreaks from 40 schools in 22 counties involving 487 students and no staff during the 2007-2008 school year. The number of cases per outbreak ranged from 3 to 39 (median, 8) during the 2008-2009 school year and 5 to 37 (median, 9) during the 2007-2008 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 2008-2009 school year, MDH received 33 reports of varicella from 12 (15%) sentinel schools. One sentinel school reported a cluster of cases that met the outbreak definition. Thirteen (39%) of 33 reported cases were included in this outbreak. The 20 cases not associated with an outbreak represent sporadic varicella incidence. In 2008, MDH received no reports of varicella cases from the 47 sentinel childcare centers or 72 sentinel family childcares.

Based on sentinel school data, an estimated 433 sporadic cases of varicella would have been expected to occur during a school year among the 877,025 total school-aged children (in Minnesota schools with more than 99 students), representing 0.05% of this population, for an incidence rate of 49.4 per 100,000 population. All sporadic cases were reported in elementary schools, which had an estimated grade level-specific annual incidence rate of 103.2 per 100,000 (433 of 419,170) for elementary school students.

MDH currently conducts zoster surveillance in all schools and selected sentinel childcare centers. During the 2008-2009 school year, MDH received 188* reports of zoster from schools in 39 counties representing 0.02% of the total school population of 915,727 students for an incidence rate of 20.5 per 100,000. Ages ranged from 4 to 18 years. By comparison, MDH had received 128 reports of zoster in 43 counties throughout Minnesota during the 2007-2008 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 (89%) are physician-diagnosed.

Vaccine supply issues have stabilized since the recent vaccine shortage. The two-dose requirement for kindergarteners and seventh graders enrolling in Minnesota schools will begin fall 2009. Providers are encouraged to administer the second dose as recommended if varicella vaccine is available.

* After publication the number of reports from schools during the 2008-2009 school year was adjusted. During the 2008-2009 school year, MDH received 131 reports of zoster from schools in 39 counties representing 0.02% of the total school population of 915,727 students for an incidence rate of 14.3 per 100,000.

• For up to date information see>> Varicella (Chickenpox)
  
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2008