After years of largely predictable norovirus waves, the emergence of a new strain might have disrupted the seasonal pattern of outbreaks from this notorious stomach bug, suggests a new report by the Centers for Disease Control and Prevention.
In previous years, the U.S. usually saw norovirus outbreaks increase around December. That marked the start of the season for the virus. But last year’s season started in October, as a new norovirus strain called GII.17 drove a record wave of outbreaks.
“Continued surveillance is needed to determine if this genotype remains the dominant genotype, as well as whether the norovirus season continues to start earlier than previous years,” wrote two of the CDC’s top norovirus researchers, in an article published this month by the agency’s Emerging Infectious Diseases journal.
Outbreaks have since slowed to usual levels. But with GII.17 now identified in more than 3 in 4 outbreaks in the U.S., its displacement of the GII.4 strain that had previously been dominant is raising questions about whether norovirus season could arrive early again.
“GII.4 viruses are the main driver for norovirus seasonality. With the decrease of GII.4 outbreaks since 2024, whether GII.17 viruses will continue to cause an earlier onset of the norovirus season cannot yet be determined,” the CDC researchers wrote.
Several other countries also saw unusually large or shifted waves of norovirus this past year, after GII.17 surged to dominance.
In England, norovirus reports reached levels more than double recent seasons. GII.17 was the most frequently reported genotype of norovirus through April, far above GII.4.
In Japan, where GII.17 previously drove a wave of infections a decade ago, local health authorities in Tokyo reported a steep wave of infectious gastroenteritis — most from norovirus — that peaked weeks later than previous seasons and remains above recent years.
Miranda de Graaf, a scientist at Erasmus University Medical Center in the Netherlands coordinating the global NoroNet network of norovirus researchers, said the Dutch nation did not see an earlier start to its norovirus season. But the country did see higher numbers of norovirus outbreaks, including a few large outbreaks in between when the usual seasonal surges occurred.
GII.4 was still being detected around the world, meaning it was possible that GII.17’s dominance could be short-lived, de Graaf said. Researchers had previously wondered if GII.17 could permanently take over, only to see GII.4 cases continue to remain dominant.
“GII.17 only replaced GII.4 last year, and there has not been another seasonal peak after that. Therefore it is not clear if these shifts will last, or if GII.17 will be the predominant strain next winter season,” de Graaf said in an email.
Benjamin Lopman, an epidemiology professor at Emory University who previously worked for the CDC’s viral diseases division, said shifts in norovirus strains happen “when new variants find ways around the immunity that people have built up” to previous strains. He said that new strains usually trigger more frequent outbreaks earlier in the season.
“I expect we’ll see outbreak patterns return to more typical levels and seasonal patterns in the coming years, though it’s always challenging to predict how these viruses will behave,” Lopman said in an email.
