Large norovirus GII.17 outbreak in a school in Northern Italy: A novel emerging sublineage within an emerging genotype?

Norovirus (NoV) is one of the leading causes of acute gastroenteritis worldwide, being responsible for millions of cases each year. We report a large NoV outbreak occurred in a Northern Italy school complex, and the genomic characterization of the outbreak strain. Stool samples from 105 subjects involved in the outbreak were collected and tested for enteric pathogens by real-time RT-PCR. The infecting NoV strain was genotyped by whole genome sequencing (WGS). Finally, a phylogenetic analysis was carried out including all whole genomes available online for the same genotype. NoV GII genogroup was detectable in 85 of 105 (81%) tested stool samples. WGS performed on 32 (32/85, 37.6%) positive samples evidenced the infecting NoV strain as belonging toGII.17[P17] genotype. The phylogenetic analysis showed that its sequences clustered with recent GII.17[P17] isolates, mostly from Europe, while they formed a separated subclade with distinct mutational patterns. Our analysis confirmed all sequenced isolates belonged to the same transmission event. The detected genotype and the mutational pattern of the outbreak strain further support the emergent role of GII.17[P17] among NoV strains worldwide and the need of its close epidemiological monitoring.