Expanded Varicella Reporting in Michigan

Background:
CDC recommends that states conduct case-based varicella surveillance. In 2005, Michigan switched from aggregate case count reporting to individual case reporting, allowing more detailed descriptive analysis of reported cases than previously possible.

Objectives:
a) To assess completeness of reporting with respect to priority data elements, and
b) To test the hypothesis that breakthrough varicella (i.e. varicella among previously vaccinated persons) is milder than that experienced by unvaccinated persons.

Methods:
Report data from calendar year 2006 were analyzed using Epi Info software version 3.3.2. Cases were reviewed and completeness proportions were calculated for selected data elements, including basic demographic information, immunization history, and approximate number of lesions, which serves as a proxy for disease severity.

Results:
A total of 5,200 cases were reported in 2006. Age was reported for 5,172 (99.5%), median age was 7 years (mean age 8.1), with a range of 1 month to 86 years.
Gender was reported for 5,133 (98.7%), and was nearly evenly split between males and females (male-to-female ratio 1.04:1). Vaccination history was reported for 4,517 (86.9%) cases. Overall, 3,490 (67%) reported a history of having received varicella vaccine. Vaccinated individuals (“breakthrough” cases) were more likely to experience mild, as opposed to severe, illness than unvaccinated persons ( p <0.0005).

Conclusions:
One year after implementation of case-based varicella reporting, case reports were highly complete for priority data elements. In addition, Michigan varicella data, representing population-based surveillance, further support the finding that breakthrough varicella is milder than disease in unvaccinated persons, previously noted in active surveillance sites, vaccine post-licensure surveillance, and outbreak investigations.