After years of largely predictable norovirus waves, a new strain may have disrupted the seasonal outbreak patterns associated with this notorious stomach bug. According to a recent report by the Centers for Disease Control and Prevention (CDC), the emergence of the GII.17 strain has led to significant changes in the timing and frequency of outbreaks.
Traditionally, the U.S. has witnessed a surge in norovirus outbreaks starting around December, signaling the onset of the virus season. However, last year's norovirus season began much earlier, in October, driven by the GII.17 strain, which caused a record-breaking wave of outbreaks. The CDC emphasizes the need for continued surveillance to ascertain whether this new genotype will remain dominant and if the norovirus season will continue to start earlier than in previous years.
Although outbreaks have recently slowed to more typical levels, the GII.17 strain is now identified in more than 75% of outbreaks in the U.S. This shift raises concerns about whether the norovirus season could experience an early onset again. Historically, GII.4 viruses have been the primary drivers of norovirus seasonality. With the decline of GII.4 outbreaks since 2024, the future role of GII.17 in determining the timing of norovirus outbreaks remains uncertain, according to leading CDC researchers.
Other countries have also reported unusually large or shifted waves of norovirus outbreaks this past year, coinciding with the rise of GII.17. In England, the reporting of norovirus cases has surged to levels more than double those of recent seasons, with GII.17 becoming the most frequently reported genotype through April. Similarly, in Japan, local health authorities in Tokyo noted a steep rise in infectious gastroenteritis cases—primarily due to norovirus—peaking weeks later than in previous years and remaining elevated compared to recent seasons.
Miranda de Graaf, a scientist at Erasmus University Medical Center in the Netherlands, leads the global NoroNet network of norovirus researchers. She noted that while the Netherlands did not experience an earlier start to its norovirus season, there were increased numbers of outbreaks, including several significant incidents occurring outside the usual seasonal peaks. Although GII.4 is still detected globally, there is speculation about whether GII.17's dominance will be short-lived.
Researchers had previously debated whether GII.17 could permanently replace GII.4, but the latter continues to show resilience. According to de Graaf, GII.17 only took over last year, and there has not yet been another seasonal peak following its rise. Thus, it remains unclear if these shifts in prevalence will persist or if GII.17 will remain the dominant strain in the upcoming winter season.
Benjamin Lopman, an epidemiology professor at Emory University with a background in the CDC's viral diseases division, explained that shifts in norovirus strains often occur when new variants circumvent the immunity built by previous infections. He anticipates that outbreak patterns will return to typical levels in the coming years, although predicting the behavior of these viruses remains challenging.
The emergence of the GII.17 strain has undoubtedly altered the landscape of norovirus outbreaks, with implications that extend beyond U.S. borders. Continued research and monitoring will be essential in understanding the dynamics of norovirus strains and their effects on seasonal patterns in the future.
