Unraveling the Transmission Dynamics of a Novel Norovirus GII.17[P17] Lineage During Two Consecutive Outbreaks in a Spanish Hospital

Norovirus outbreaks in healthcare settings pose significant challenges to infection prevention and control (IPC). To prevent and control such outbreaks efficiently, identifying sources and transmission clusters (TCs) is crucial but often limited by traditional outbreak investigations. Here, we examined two consecutive hospital norovirus outbreaks employing a genomic epidemiology approach to elucidate transmission dynamics and guide IPC strategies. Stool samples of 54 symptomatic patients were analyzed with different diagnostic methods, and 26 norovirus-positive samples underwent metagenomic next-generation sequencing (mNGS) for phylodynamic and phylogenetic analyses. All infections belonged to the novel GII.17[P17] lineage, circulating globally since 2023/2024. LiquidArray® outperformed fluorescence immunoassay (FIA, 28.6%) and RT-PCR (85.7%) with FIA's low sensitivity leading to missed cases highlighting the need for molecular confirmation for accurate outbreak management. Genomic analysis revealed multiple introductions, with two TCs identified in Outbreak-1 and one in Outbreak-2, as well as inter-hospital-unit spread. Reconstruction of transmission trees indicated sustained person-to-person spread with 0-3 unobserved intermediate cases in both outbreaks. Identical sequences in patients without clear epidemiological links suggested possible fomite transmission. These analyses provided key insights into infection sources and TCs that would have remained unknown using epidemiological investigations alone, supporting more targeted IPC resource allocation and intervention strategies.