Noroviruses represent one of the leading causes of outbreaks and sporadic acute gastroenteritis (AGE) cases across all age groups. Although the GII.4 norovirus has been identified as the primary cause of most AGE outbreaks, the transient predominance of other genotypes has been reported globally. In this study, we describe a multi-province AGE outbreak caused by a potential new lineage of norovirus GII.17[P17], which has been recently detected at a high incidence in the United States and Europe. An amino acid analysis of the major viral capsid protein revealed several substitutions in the hypervariable region compared to strains circulating in the mid-2010s, which could play a key role in immune evasion. This is the first report of the detection of these viruses in the Southern Hemisphere, underscoring the importance of maintaining active genomic surveillance in the context of increasing numbers of acute gastroenteritis outbreaks.

Norovirus (NoV) is a leading cause of acute gastroenteritis (AGE) in young children worldwide. Following the introduction of universal rotavirus (RVA) vaccination in Argentina in 2015, the role of NoV in pediatric AGE warrants evaluation. This study aimed to assess the prevalence, clinical characteristics, and molecular diversity of NoV in children under five years of age, comparing the periods before and after RVA vaccine implementation. Methods: A descriptive observational study was conducted in two pediatric hospitals in Argentina. Stool samples were obtained from both outpatient and hospitalized children presenting with acute gastroenteritis (AGE) during two distinct one-year periods: 285 samples from the pre-vaccination period (2011–2012) and 212 samples from the post-vaccination period (2019–2020). NoV, RVA and other viral enteropathogens were detected by RT-qPCR or immunoassay. Positive NoV samples were genotyped by Sanger sequencing of the ORF1/ORF2 junction. Results: NoV was detected in 30.1% (86/285) and 23.5% (50/212) of cases in the pre- and post-vaccination periods, respectively. Children under two years of age and inpatients had significantly higher NoV detection in both periods. NoV mono-infections were more frequent in post-vaccination period (72% vs. 50%). NoV GII predominated in both periods, with increased genotype diversity observed post-vaccination, including GII.3[P12], GII.4 Sydney[P16], GII.6[P7], and GII.2[P16]. Conclusions: NoV remains a major cause of pediatric AGE in Argentina, particularly in children under two years old. Although NoV prevalence did not increase after RVA vaccine introduction, its clinical relevance persists. Continued molecular surveillance is essential to monitor genotype dynamics and implement prevention strategies.