Norovirus GII.17 Takes the Lead in Recent US Outbreaks

Norovirus GII.17 accounted for 75% of outbreaks in the US in the 2024-2025 season, outpacing outbreaks caused by the GII.4 norovirus, according to new data published in Emerging Infectious Diseases.

Noroviruses are classified into 10 groups known as GI-GX, but most norovirus outbreaks are caused by viruses in genogroups GI and GII, wrote Leslie Barclay, microbiologist at the CDC, Atlanta, and Jan Vinje, PhD, also of the CDC. The norovirus season on the US has been defined as September 1 of 1 year through August 31 of the following year, they noted.

Norovirus GII.17 accounted for < 10% of outbreaks in the US 2022-23 season, but rose to 75% during the 2024-25 season, passing the number of GII.4 outbreaks.

During the 2022-23 season, GII.17 accounted for only 7.5% of all norovirus outbreaks, whereas GII.4 accounted for nearly half (48.9%).

The following season (2023-2024), GII.17 accounted for approximately one third of norovirus outbreaks (34.3%), whereas GII.4 accounted for 27.7%. In the 2024-25 season, GII.17 accounted for 75.4% of norovirus outbreaks, whereas GII.4 accounted for 10.7%.

In addition, norovirus activity peaked earlier in 2024-25 (accompanying the rise in GII.17) compared to previous seasons. The 2024-25 season peaked in January 2025, vs peak periods in February 2023 and March 2024 when GII.4 still dominated.

The researchers observed no regional differences in the distribution patterns of the GII.4 or GII.17 outbreaks.

The study findings were based on data from CaliciNet, a national laboratory-based surveillance network. “Norovirus surveillance plays a crucial role in detecting and monitoring emerging strains, serving as an early warning system that enables rapid response to outbreak investigations and timely implementation of interventions and prevention strategies,” the authors noted.

However, the results highlight the need for improved norovirus surveillance to detect and monitor emerging strains, they added. Specifically, more sequencing analysis is needed to compare cross-protective neutralizing antibodies of GII.17 with those of GII.4 to help determine whether GII.17 is likely to continue as the dominant norovirus genotype, the authors concluded.

How Shifting Strains Hit Clinical Practice 

Tracking dominant norovirus strains is important for public health and clinical reasons, as dominant strains often drive regional or global outbreaks, said Rebecca Yee, PhD, D(ABMM), chief of microbiology and assistant professor of pathology at the George Washington School of Medicine and Health Sciences, Washington, DC, in an interview.

“Tracking specific strains allows public health agencies to perform surveillance and trace sources, monitor spread, and implement control measures more effectively,” said Yee, who was not involved in the study. “As the virus undergoes genetic changes, monitoring viral genomics allows researchers to study emerging variants that may have higher transmissibility or altered virulence in the human host,” she said. Since some molecular assays are designed to detect specific genotypes, understanding shifts in circulating strains helps clinical laboratories anticipate the need for test updates to maintain diagnostic accuracy, and these insights also will be valuable for vaccine development, Yee said.

The recent shift in genotype distribution is not entirely surprising, given norovirus’s reputation for rapid genetic evolution through acquisition of mutations and genetic recombination with other strains, Yee told Medscape Medical News.

“In theory, these genetic changes allow new variants or strains to harbor characteristics that can make the virus more transmissible, resistant to existing immunity, capable of causing more severe disease and leading to periodic shifts in dominant strains is not an exception. However, the timing or speed of a shift can still be surprising, especially if it leads to unusually large or early-season outbreaks such as what we are seeing with the recent norovirus cases,” Yee said.

Studies using dominant circulating strains and genetic variants can be performed to evaluate cross-protection and vaccine effectiveness, said Yee. “As new norovirus variants emerge, it is critical to assess their clinical severity, particularly in vulnerable populations; these variants may exhibit changes in viral attachment, immune evasion, or modulation of host responses that can enhance their pathogenicity,” she emphasized.

“Furthermore, diagnostic assays must be evaluated to ensure they remain sensitive to emerging variants. Together, these efforts will help inform vaccine design, clinical management, and testing strategies,” Yee said.

This study received no outside funding. The researchers had no financial conflicts to disclose. Yee disclosed receiving research support from BioMerieux.