GII.17[P17] Norovirus Resurge in Beijing, China, 2024-2025.

Noroviruses are a major cause of acute gastroenteritis worldwide. In Beijing, norovirus GII.17 outbreaks, which dominated in 2014-2015 (66.7%), surged again from 1.3%-12.2% (2015-2024) to 81.7% (2024-2025), with reported outbreaks during July 2024-February 2025 dramatically exceeding the prior year (301 vs. 103). From 2014 to 2025, 249 GII.17 outbreaks were identified, comprising 107 Kawasaki308 (1525 cases) and 142 Romania-2021 (1840 cases) based on phylogenetic analysis. The proportion of outbreak settings differed for Romania-2021 and Kawasaki308: kindergartens (35.2% vs. 37.4%), primary schools (46.5% vs. 29.0%), middle schools (5.6% vs. 12.1%) and others (12.7% vs. 21.5%). Romania-2021 cases were younger and had more vomiting than Kawasaki308 cases. Phylogenetic analysis revealed that novel GII.17 strains dominated in Beijing in 2024-2025 clustered with contemporaneous global strains, forming a distinct subcluster (New_2023-2025) within New (Romania-2021 like) cluster. This subcluster exhibited unique amino acid substitutions in VP1, VP2 and non-structure proteins compared to other clusters and New subclusters. Our findings confirm the dominance of Romania-2021 like strains during 2024-2025 in Beijing. Differences in symptom profiles and affected age groups between Kawasaki308 and Romania-2021, and unique amino acid substitutions may help explain the rapid predominance of the emerging cluster.