Impact of the 2024 Council of State and Territorial Epidemiologists’ Varicella Case Definition and Classification on Varicella Cases Reported in Minnesota

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Abstract
An updated varicella case definition and classification from the Council of State and Territorial Epidemiologists became effective in 2024, emphasizing laboratory confirmation by requiring that confirmed cases be laboratory confirmed or epidemiologically linked to a laboratory-confirmed case, in addition to meeting clinical criteria. We assessed the impact of the changes using varicella reports in Minnesota from January 1, 2024, through May 31, 2025. Of 737 reports, 61.2% (n = 451) met the new definition and 61.9% (n = 456) met the old definition; 12.1% (n = 89) changed classifications. The proportion of cases classified as confirmed decreased by 17.5%, while those classified as probable increased by 28.0%; changes were primarily driven by the requirement to link reports to a laboratory-confirmed case. Reports not classified (ie, undefined; 26.9%; n = 198) mostly lacked rash descriptions. The updated definition improves jurisdictions’ ability to classify cases accurately, thus strengthening national surveillance by reflecting current varicella epidemiology. Improved reporting of rash description, epidemiologic information, and increased laboratory testing are needed to improve surveillance.

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