Cynthia Kenyon

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.

Background: Older individuals (≥65 years) are at greatest risk of severe influenza requiring hospitalization. Since 2009, influenza-associated hospitalization rates during A(H1N1)pdm09-predominant influenza seasons have been lower than during A(H3N2)-predominant seasons. Methods: Using laboratory-confirmed influenza hospitalization rates from US population–based surveillance, we investigated the relative A(H1N1)pdm09 to A(H3N2) hospitalization rate ratios by patient year of age and birth cohort from 2010 through 2025. Results: Results suggest partial protection against A(H1N1)pdm09 hospitalizations relative to A(H3N2) hospitalizations among patients born before 1945 (pre-1945 birth cohorts) and among individuals born during 1994 through 2009. Impact of partial protection against A(H1N1)pdm09-associated hospitalizations has diminished over time among older adults, contributing to elevated influenza hospitalization rates during the 2024–2025 influenza season. Conclusions: Our findings suggest that influenza A hospitalization rates may increase during future influenza seasons when influenza A(H3N2) and A(H1N1) viruses circulate or following the emergence of novel A(H1N1)pdm09-like viruses.