Changes in Norovirus Patterns Observed
After years of fairly regular norovirus outbreaks, a new strain’s emergence could be altering the typical seasonal pattern, according to a report from the Centers for Disease Control and Prevention.
Historically, norovirus outbreaks in the U.S. ramped up around December, signifying the start of the season. However, last year, the season kicked off in October due to a strain known as GII.17, which sparked an unprecedented wave of infections.
Two prominent CDC researchers pointed out in a recent journal article that ongoing monitoring is crucial to see if this new variant remains dominant and whether the outbreaks continue to start earlier than in the past.
Currently, outbreaks have slowed down to expected levels. Yet, with GII.17 detected in more than 75% of U.S. outbreaks, its rise over the previously dominant GII.4 strain raises concerns about the potential for another early outbreak season.
The CDC experts noted that GII.4 has traditionally driven the seasonal patterns of norovirus. With its decline since 2024, it’s uncertain if GII.17 will maintain the trend of earlier onset.
Interestingly, other countries have also experienced irregular norovirus activity, particularly with GII.17 taking the lead. Reports from England indicated levels more than double those of recent years, with GII.17 being the most frequently noted variant through April, significantly outnumbering GII.4.
In Japan, a country where GII.17 had caused a significant outbreak a decade ago, authorities reported a peak in gastroenteritis cases—mainly from norovirus—happening later than usual and remaining high compared to recent years.
A researcher from the Netherlands, Miranda de Graaf, mentioned that while the Dutch didn’t experience an earlier season, there were still more frequent norovirus outbreaks, with some larger ones occurring between normal seasonal peaks.
Despite GII.4 still being detected around the globe, de Graaf cautioned against assuming that GII.17’s dominance will last long term. Previous expectations of GII.17 taking over fully were met with the continued presence of GII.4.
She noted in an email that GII.17 has only just replaced GII.4, and it remains unclear whether this change will persist into the next winter season.
Benjamin Lopman, an epidemiologist at Emory University, shared insights on how shifts in norovirus strains can happen when new variants find ways to evade the immunity that populations have built up. He pointed out that these newer strains tend to cause more frequent outbreaks earlier in the season.
Looking ahead, Lopman expressed hope that outbreak patterns might revert to more typical levels in future years, though he acknowledged the difficulty in predicting viral behavior.
