Abstract Title
Norovirus GII.17 Surpasses GII.4 Outbreaks in the United States
Presenter
Leslie Barclay, Centers for Disease Control and Prevention
Co-Author(s)
Anna M. Montmayeur, Jan Vinjé, Preeti Chhabra. Division of Viral Diseases, Centers for Disease Control and Prevention, Atlanta, GA, USA
Abstract Category
Epidemology
Abstract
Introduction: Noroviruses have been the leading cause of epidemic acute gastroenteritis (AGE) in the United States, with the majority of outbreaks over the past 20 years attributed to GII.4 viruses.
Methods: Epidemiologic and genotypic data from AGE outbreaks (all ages) from September 1, 2022—June 30, 2025, were downloaded from CaliciNet and analyzed. Genotypes were grouped into 3 categories: GII.17, GII.4 (including GII.4 Sydney, GII.4 San Francisco, and GII.4 Wichita), and other genotypes (all other GI and GII genotypes). Outbreak season was defined as September – August. The complete genome of a selection of GII.17 strains was sequenced and analyzed.
Results: During the 2022–23 season, GII.17 accounted for 7.5% of outbreaks, while GII.4 comprised 48.9%. In 2023–24 season, GII.17 rose sharply to 34.2%, surpassing GII.4 (27.6%). By the 2024–25 season (truncated to June 2025), GII.17 dominance was clear, responsible for 74.5% of outbreaks, while GII.4 declined to 11.6%. Monthly data showed GII.17 surpassed GII.4 beginning in April 2024 and remained predominant through May 2025. Seasonality patterns shifted, with the norovirus season starting in October 2024 and peaking in January 2025, earlier than in previous seasons. Phylogenetic analysis showed that the majority of GII.17 sequences clustered with strains first detected in Romania in 2021, with only a few exceptions.
Conclusions: In 2024, GII.17 emerged as the predominant norovirus genotype in US outbreaks, surpassing GII.4 for the first time. Our data highlight the importance of ongoing standardized surveillance for early detection of emerging norovirus genotypes.
Methods: Epidemiologic and genotypic data from AGE outbreaks (all ages) from September 1, 2022—June 30, 2025, were downloaded from CaliciNet and analyzed. Genotypes were grouped into 3 categories: GII.17, GII.4 (including GII.4 Sydney, GII.4 San Francisco, and GII.4 Wichita), and other genotypes (all other GI and GII genotypes). Outbreak season was defined as September – August. The complete genome of a selection of GII.17 strains was sequenced and analyzed.
Results: During the 2022–23 season, GII.17 accounted for 7.5% of outbreaks, while GII.4 comprised 48.9%. In 2023–24 season, GII.17 rose sharply to 34.2%, surpassing GII.4 (27.6%). By the 2024–25 season (truncated to June 2025), GII.17 dominance was clear, responsible for 74.5% of outbreaks, while GII.4 declined to 11.6%. Monthly data showed GII.17 surpassed GII.4 beginning in April 2024 and remained predominant through May 2025. Seasonality patterns shifted, with the norovirus season starting in October 2024 and peaking in January 2025, earlier than in previous seasons. Phylogenetic analysis showed that the majority of GII.17 sequences clustered with strains first detected in Romania in 2021, with only a few exceptions.
Conclusions: In 2024, GII.17 emerged as the predominant norovirus genotype in US outbreaks, surpassing GII.4 for the first time. Our data highlight the importance of ongoing standardized surveillance for early detection of emerging norovirus genotypes.