UNMC_Acronym_Vert_sm_4c
University of Nebraska Medical Center

Increasing Predominance of Norovirus GII.17 over GII.4, United States, 2022–2025

CDC

Norovirus is the leading cause of acute gastroenteritis outbreaks in the United States (1). Genetically, noroviruses are classified into 10 genogroups (GI–GX) and further into 48 genotypes and 60 P-types (2). Most outbreaks are caused by genogroup GI and GII viruses. During 2011–2024, GII.4 viruses have caused >50% of US outbreaks each season (defined as September 1 of one year through August 31 of the next) (3).

Laboratory surveillance of norovirus in the United States is conducted through CaliciNet, a network of public health laboratories from local, state, and federal agencies (4). As previously reported (5), several countries, including the United States, observed an increase in GII.17 cases and outbreaks during the 2023–24 season. Initially, GII.17 outbreaks in the United States remained below GII.4 outbreak numbers. We present an updated analysis on the increase of GII.17 outbreaks since September 2022.

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