Emergence and predominance of GII.17[P17] noroviruses in Brazil, 2023-2024.

Norovirus is a leading cause of acute gastroenteritis (AGE) worldwide, affecting all age groups. In the present study, we investigated fecal samples from medically attended AGE patients received from ten Brazilian states collected during 2023 and 2024. Norovirus GI and GII were detected and quantified using RT-qPCR, and norovirus-positive samples underwent genotyping through sequencing the ORF1/2 junction region. During the two-year period, norovirus prevalence was 22.5%. GII genotypes were predominant, accounting for 85.6% of the genotyped samples. By genotyping, we detected the circulation of 19 norovirus genotypes, of which the emergence of GII.17[P17] in 2023 and its predominance in 2024. Comparison of cycle threshold (Ct) values indicated that the median viral load was significantly higher for norovirus GII.17 compared to GII.4 (20.9 and 22.3, respectively (p = 0.0493). Additionally, we showed that GII.17 patients were older compared to GII.4 patients. Our study is the first in South America to demonstrate replacement of the previous dominant genotype (GII.4 Sydney[P16]) to GII.17[P17] in 2024, currently disseminated worldwide.