It feels like everyone has a stomach bug. A new norovirus strain might be to blame, CDC data suggests

A new strain of norovirus now makes up a majority of outbreaks nationwide, according to data from the Centers for Disease Control and Prevention. The new strain’s emergence, found spreading throughout the country and on cruise ships, offers a possible explanation for the past month’s steep wave of infections from the stomach bug. 

Scientists call this winter’s dominant norovirus strain GII.17[P17]. Samples from patients in nearly 7 in 10 norovirus outbreaks have tested positive for the new strain so far this season, according to data from a network of public health laboratories called CaliciNet. 

“It is too early to tell if this strain is associated with more severe norovirus disease but likely lower population immunity is the reason for the early surge of an otherwise seasonal increase of this virus,” Jan Vinjé, head of the CDC’s CaliciNet system, told CBS News in an email.

All previous waves of norovirus in the U.S. since 2012 have been dominated by a different strain that scientists call GII.4, said Vinjé. This season, only around 7% of norovirus outbreaks have been linked to that GII.4 strain.

Vinjé said that the GII.17 strain is behind outbreaks “everywhere” across the country this season of the highly contagious virus, including on cruise ship outbreaks.

The CDC tracked at least six norovirus outbreaks on vessels last month, marking the most reported in the month for over a decade. 

Norovirus is the most common cause of foodborne illness in the U.S. and almost always climbs around the country starting during the winter months. 

Many cases resolve on their own without needing to be tested or treated by doctors, making it impossible to know exactly how many Americans are sickened. However, virtually all data tracking the virus suggest norovirus is at peak levels throughout the country.

Rates of positive norovirus tests from a different CDC network of laboratories have reached the highest weekly levels on record in years. Norovirus rates in figures from testing company BioFire Diagnostics are also now above previous seasonal peaks. 

In wastewater, samples collected by WastewaterSCAN have been “high” for norovirus across nearly all sites monitored by the firm across the country. 

While versions of GII.17 have been circulating for several decades, only in recent years has the strain driven large waves of infections. Some parts of Europe last year reported a “sudden rise” in GII.17. Japan and China reported their first waves from the strain after 2014.

The CDC reported it had seen a rise in GII.17 cases last season, though it remained a minority of all norovirus outbreaks at the time in the U.S.

Is GII.17 causing more or worse norovirus cases?

Additional studies will also be needed to tease out whether GII.17 is leading to a difference in the severity of norovirus infections. Some previous changes to the virus have emerged without causing new or significantly worse symptoms. 

“What we are seeing has happened before for noroviruses. Noroviruses are very diverse,” said Marion Koopmans, head of the Erasmus MC Department of Viroscience.

Koopmans, who oversees a global consortium of norovirus researchers, cautioned that data tracking norovirus cases still has gaps around the world. More studies are also needed to prove if there is an actual increase in illnesses being driven by the strain.

“There is insufficient surveillance (in my view) to really make the case that this is causing an increase in the number of cases, as we have a lot of norovirus infections each winter and the surveillance is rather patchy,” said Koopmans.

When the latest GII.4 strain, called the Sydney variant, emerged in 2012, initial data suggested it had driven an increase in outbreaks. The CDC later reported data from hospitals suggesting it had not actually led to significantly more or worse norovirus infections.

“Thus, the observed increase in outbreak activity during the 2012–13 season likely represents random seasonal and state variation rather than a direct result of the emergence of GII.4 Sydney,” CDC scientists concluded at the time.

Health officials in Wisconsin said earlier this month that they had seen a record-high number of norovirus outbreaks, but told CBS News they do not believe it is driving a major difference in symptoms or length of illness.

“It does not change any recommendations about prevention or spread. It just means we have a robust outbreak season and therefore an opportunity to really emphasize the importance of handwashing, staying home while sick and until at least 48 hours after recovery,” said Elizabeth Goodsitt, spokesperson for the Wisconsin Department of Health Services, in an email.