What Is Behind Norovirus Spikes This Season?

Norovirus cases continue to rise in the United States this winter, at levels higher than the same time period in previous years, according to the latest data from the Centers for Disease Control and Prevention (CDC).

Current data from the CDC’s NoroSTAT monitoring system show 495 reported outbreaks during the period from August 1, 2024, to December 11, 2024, compared with 363 outbreaks during the same period last year. In addition, the total number of norovirus outbreaks in the current season are higher than those reported in the seasonal years: 2012-2020 and 2021-2024.

Circulating strains of norovirus change over time, which can affect disease burden and potential disease severity, said Sara Mirza, MD, an epidemiologist in the CDC’s Division of Viral Diseases, in an interview.

The numbers for the 2024 norovirus season (considered approximately November to April) have reached or exceeded the case numbers seen before the COVID-19 pandemic, Mirza said.

The increase in cases may be caused in part by a new predominant strain of norovirus. “For the fall/winter of 2024-2025 season, genogroup 2, genotype 17, known as GII.17, has become the most detected genotype (strain) in the US among laboratory confirmed outbreaks reported to CDC,” said Mirza. “At this time, there is no indication that GII.17 causes more severe illness or affects one population more than another, but we are continuing to conduct surveillance to assess,” she added.

Clinical Takeaways

“Norovirus affects all ages, but young children and older adults are most at risk from more severe outcomes,” Mirza told Medscape Medical News.

“Clinicians treating older patients for acute gastroenteritis should be aware of these elevated risks and be sure to include norovirus as a potentially serious diagnosis, particularly in vulnerable patients with other diseases and those living in congregate settings, such as nursing homes,” she said.

When treating a patient with norovirus during an outbreak, use soap and water for hand hygiene after caring for patients with suspected or confirmed norovirus gastroenteritis, said Mirza. If norovirus infection is suspected, PPE use is recommended for individuals in the patient care area, she added. 

Although state, local, and territorial health departments are not required to report individual cases of norovirus to the CDC, healthcare providers are encouraged to report all outbreaks of acute gastroenteritis, including suspected outbreaks of norovirus, to the appropriate state, local, or territorial health department, said Mirza. “Health departments are encouraged to report all suspected and confirmed norovirus outbreaks through the National Outbreak Reporting System and CaliciNet,” she added.

“Infection control measures, such as thorough hand washing, cleaning and disinfecting surfaces with bleach, and patient isolation and contact precautions in congregate or healthcare settings are the best ways to prevent norovirus and keep it from spreading to others,” Mirza told Medscape Medical News.

Remind patients that alcohol-based hand sanitizer is ineffective against norovirus, because the virus’s protective protein shell prevents the alcohol from penetrating and inactivating the virus, Mirza emphasized. “Soap and water work to remove germs from hands,” she said.

Cruise Ship Considerations

Cruise ships continue to be sources of increased risk for norovirus, according the CDC. The CDC’s Vessel Sanitation Program (VSP) was created to help the cruise industry prevent public health issues such as norovirus outbreaks, and to provide guidance for actions to take in the event of outbreaks. 

For example, the most recently reported outbreak of norovirus on a cruise ship reported to the VSP was January 4, 2025, and occurred on a Holland America cruise from December 30, 2024, through January 8, 2025. Overall, 4.0% of passengers and 1.0% of crew members reported illness. Following VSP guidance, the ship reported increased cleaning and disinfection procedures and the collection of stool specimens for testing, and isolation of ill passengers and crew.

Clinical Perspective

In clinical practice, the number of norovirus cases is significantly exceeding previous years, and the trend seems to be consistent nationwide, said David J. Cennimo, MD, associate professor of medicine & pediatrics at Rutgers New Jersey Medical School, Newark, New Jersey, in an interview.

“Norovirus is incredibly contagious and spreads very quickly, which is how you get entire cruise ships infected at once,” he said. Norovirus is notoriously difficult to disinfect or kill, he added.

One possible contributor to the surge in cases is increased travel, especially during the holiday season, when people are coming together and sharing food, Cennimo noted. “We have seen many infections such as pneumonia return to levels approaching the period before the COVID-19 pandemic,” Cennimo told Medscape Medical News. 

For norovirus prevention, strict attention to sanitation and handwashing is a must at home or when traveling, said Cennimo. For clinicians, it is important to report outbreaks of GI illness so appropriate control measures can be taken, he said.

Visit the CDC’s website on norovirus prevention for more information. 

Mirza and Cennimo had no financial conflicts to disclose.