Varicella and Zoster, 2010

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 school 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 fifth full year of surveillance.

Five cases of critical illness, but no deaths, due to varicella were reported. Four of the 5 were hospitalized for 2 to 15 days. Complications included pneumonia and bacterial super-infection. One case had an underlying medical condition and recent history of treatment with immunosuppressive drugs. The other cases had no or unknown underlying conditions and were not known to be immunosuppressed. Four cases had not received varicella-containing vaccine; 2 were too young to have received vaccine, 1 was not vaccinated due to parental refusal, and 1 was not vaccinated and had a history of varicella disease at 33 days of age. Vaccination history for the other case, age 70 years, was unknown.

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 2010-2011 school year, MDH received reports of outbreaks from five schools in five counties involving 31 students and no staff. By comparison MDH received reports of outbreaks from 20 schools in 16 counties involving 180 students and 2 staff during the 2009-2010 school year. The number of cases per outbreak ranged from 5 to 11 (median, 5) during the 2010-2011 school year compared to 4 to 26 (median, 9) during the 2009-2010 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 2010-2011 school year, 78 of the 79 sentinel schools that participated in the 2009-2010 school year participated again. 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; however, sentinel sites have been requested to also report possible breakthrough infection that may present atypically. During the 2010-2011 school year, 18 cases were reported from 11 schools. None of the schools reported a cluster of cases that met the outbreak definition. Based on these data, an estimated 407 sporadic cases of varicella would have been expected to occur during a school year among the 870,941 total school-aged children (for schools with >99 students), representing 0.05% of this population, for an incidence rate of 46.7 per 100,000 population. Most cases occurred among elementary school students, with an estimated incidence rate of 82.4 per 100,000 (392 of 475,777).

Case-based reporting of varicella in all child care settings was initiated in February 2010. For the remaining 11 months of 2010, 111 cases were reported. Cases ranged in age from 4 months to 11 years. Ninety-eight (88%) were <6 years of age.

Updated Monday, April 16, 2012 at 09:40AM