In previous years, the U.S. usually saw norovirus outbreaks increase around December.
But last year’s season started in October, as a new norovirus strain called GII.17 drove a record wave of outbreaks.
Several other countries also saw unusually large or shifted waves of norovirus this past year, after GII.17 surged to dominance.
GII.17 was the most frequently reported genotype of norovirus through April, far above GII.4.
But the country did see higher numbers of norovirus outbreaks, including a few large outbreaks in between when the usual seasonal surges occurred.
The Centers for Disease Control and Prevention recently released a report that suggests the emergence of a new strain of norovirus may have upset the seasonal pattern of outbreaks from this infamous stomach bug after years of largely predictable waves.
Some years ago, the U. S. . Typically, norovirus outbreaks peaked in December. That was when the virus’s season began. However, a new norovirus strain known as GII began the season in October of last year. 17 sparked a record-breaking outbreak wave.
In a paper released this month by the CDC’s Emerging Infectious Diseases journal, two of the agency’s leading norovirus researchers stated that “continued surveillance is needed to determine whether this genotype remains the dominant genotype, as well as whether the norovirus season continues to start earlier than previous years.”.
Since then, outbreak rates have decreased to normal. GII, though. In over three out of four U.S. outbreaks, 17 have now been identified. A. the way it replaced the GII. Concerns are being raised about the possibility of an early norovirus season due to four strains that were previously dominant.
“GII,”. The primary cause of norovirus seasonality is four viruses. whenever GII declines. four outbreaks, whether GII, since 2024. It’s still unclear whether 17 viruses will cause the norovirus season to start earlier,” the CDC researchers wrote.
Following GII, several other nations experienced abnormally large or shifted norovirus waves in the past year. 17 sprang to power.
Reports of norovirus more than doubled in England in recent seasons. GII. Up until April, norovirus genotype 17 was reported far more often than GII. 4.
where GII is located in Japan. A steep wave of infectious gastroenteritis, primarily from norovirus, peaked weeks later than previous seasons and is still higher than in recent years, according to local health authorities in Tokyo, which was the cause of a wave of infections ten years ago.
The Netherlands did not experience an earlier start to its norovirus season, according to Miranda de Graaf, a scientist at Erasmus University Medical Center in the Netherlands who oversees the global NoroNet network of norovirus researchers. Nonetheless, there were more norovirus outbreaks across the nation, including a few significant outbreaks that happened in between the typical seasonal spikes.
GII. Since 4 was still being found globally, GII might be involved. According to de Graaf, the dominance of 17 might not last long. Before, researchers had questioned whether GII. 17 could take control for good, only to see GII. Four cases are still in the lead.
“GII. GII was replaced only by 17. 4 last year, and since then, there hasn’t been another seasonal high. As a result, it is unclear whether GII or these changes will continue. “Next winter, strain 17 will be the most common strain,” de Graaf wrote in an email.
According to Benjamin Lopman, a professor of epidemiology at Emory University and a former employee of the CDC’s viral diseases division, changes in norovirus strains occur “when new variants find ways around the immunity that people have built up” to earlier strains. According to him, outbreaks that occur earlier in the season are typically caused by new strains.
Lopman wrote in an email, “It’s always difficult to predict how these viruses will behave, but I expect we’ll see outbreak patterns return to more typical levels and seasonal patterns in the coming years.”.
