The Largest Outbreak of Acute Gastroenteritis of Mixed Norovirus Genogroups in the Coast of São Paulo State, Brazil.

During the Brazilian summer, from 29 December 2024, to 6 March 2025, a large cluster of acute gastroenteritis (AGE) outbreaks was reported along the coast of São Paulo State, Brazil, peaking in January 2025. Overall, 55 outbreaks involving 755 cases were officially notified, while more than 76,000 medical consultations for AGE were recorded across the region during the same period. A total of 50 stool samples were analyzed by RT-qPCR for group A rotavirus (RVA) and norovirus (NoV). NoV was detected in 27 samples (54.0%), confirming it as the main etiological agent, while RVA was identified in one sample (2.0%). Among NoV-positive cases, genogroup II (GII) predominated (59.0%), followed by genogroup I (GI) (19.0%) and mixed infections (22.0%). Genomic sequencing successfully genotyped 23 strains (95.8%), revealing six distinct genotypes. The recombinant GII.17[P17] was predominant (48.0%), followed by GI.3[P3], GI.3[P13], GI.5[P5], GII.4 Sydney_2012[P16], GII.3[P30], as well as mixed infections. No enteric viruses were detected in drinking water. However, seawater samples showed high concentrations of NoV GI and GII (up to 104 GC L-1) at beaches unsuitable for bathing. Wastewater surveillance revealed high viral loads, particularly NoV GII (up to 108 GC L-1), consistent with reported cases. To our knowledge, this is the first report in Brazil of a NoV-associated AGE outbreak investigated through an integrated approach combining clinical, environmental, and epidemiological surveillance data. Findings highlight genotype diversity and reinforcing the importance of integrated surveillance.