Health experts are pointing to a new and more infectious strain of norovirus as the driver behind a significant and early surge in cases of the stomach bug in the United States. The development serves as a warning for public health officials in the UK and elsewhere, as the ultra-contagious virus often spreads globally.
A New, More Infectious Variant Takes Hold
Doctors were puzzled when norovirus infections began to climb sharply in November, weeks ahead of the typical seasonal peak which usually follows festive gatherings. The culprit has now been identified as a strain dubbed GII.17, which became dominant in the US last year and is fuelling another major season.
According to the latest data from the US Centers for Disease Control and Prevention (CDC), 11.73 percent of all norovirus tests were positive in the week ending November 29. This marked a notable increase from 9.51 percent a month earlier and a significant jump from the 8.7 percent positivity rate seen at the same time in 2023, before the new strain took hold.
Widespread Impact and Rising Concerns
While test positivity dipped slightly to 9.4 percent in the week ending December 6, wastewater surveillance data indicates virus levels continue to rise nationwide. This metric is crucial as many infected individuals do not seek formal testing, meaning true case numbers are likely far higher than official reports suggest.
The CDC warns that the GII.17 strain causes approximately 50 percent more infections than other variants and is now responsible for at least 75 percent of norovirus cases in the US. Fortunately, there is no current evidence that it leads to more severe illness or higher mortality rates.
Norovirus, often called the 'winter vomiting bug', leads to around 21 million illnesses, 109,000 hospitalisations, and 900 deaths annually in the US. It also triggers roughly 2,500 outbreaks every year in settings like schools and hospitals, with 2,600 recorded last year.
School Closures and Expert Warnings
The real-world disruption caused by the virus is already evident. At least two elementary schools in the US, in Massachusetts and New Jersey, have been forced to close for deep cleaning following suspected norovirus outbreaks affecting hundreds of students.
Infectious diseases expert Dr Scott Roberts of Yale University stated, 'Whenever there's a new strain, it's really easy for it to start spreading rapidly, and that's what we saw last year.' He added that while current levels have not yet reached the severe peak of the previous year, activity is 'certainly going up' and urged the public that 'now is the time to be vigilant'.
Wastewater monitoring shows norovirus levels are currently 'medium' nationwide in the US, with 'high' activity on the West Coast and rising trends in most regions.
The emergence of GII.17, which originated from a genetic swap between two other strains in Romania in 2021, coincides with increased activity of other infectious diseases. Health authorities in South Carolina are managing a measles outbreak, while influenza activity is rising in several US states and has already surged early in the UK. This confluence underscores the ongoing challenge of managing seasonal viral threats.
