New norovirus strain drives early surge in U.S. cases, health officials say

A new strain of norovirus, labeled GII.17, is driving a surge in cases this winter and has helped push the United States into an unusually early peak for the ultra-contagious stomach bug. Doctors say the virus, known for causing vomiting so intense it can crack ribs, is spreading faster than in previous seasons as health officials monitor a shifting pattern of transmission ahead of the holidays.

In the latest weekly snapshot, the CDC reported that 11.73 percent of norovirus tests were positive in the week ending Nov. 29, up from 9.51 percent a month earlier and well above the 8.7 percent seen at the same point in 2023, before GII.17 took hold in the country. Wastewater data tracked by WastewaterSCAN show lingering increases in norovirus levels nationwide, with regional differences and recent upticks in several areas. While many people who contract norovirus do not seek testing, health authorities say the positivity trends and wastewater signals together point to a true rise in activity.

The CDC notes that GII.17 now accounts for roughly three-quarters of identified infections in the United States and is associated with about 50 percent more cases than other circulating strains. There is currently no evidence that the variant causes more severe disease or higher mortality, but experts say the higher transmissibility can still drive a larger overall burden. Officials estimate that about 21 million Americans are infected with norovirus each year, resulting in roughly 109,000 hospitalizations and about 900 deaths. Norovirus outbreaks occur in about 2,500 settings annually, including schools, hospitals and other public venues, and last season saw roughly 2,600 reported outbreaks in such places. Symptoms typically appear 12 to 48 hours after exposure and include vomiting, diarrhea, stomach cramps, nausea and fever. Most people recover within one to three days, but the illness can be more serious for young children, the elderly and people with weakened immune systems.

Dr. Scott Roberts, an infectious diseases expert at Yale University, cautioned that a new strain can ignite a rapid spread. He told TODAY that the emergence of a novel variant often accelerates transmission and that officials are watching closely for any changes in disease severity. “Whenever there’s a new strain, it’s really easy for it to start spreading rapidly, and that’s what we saw last year,” Roberts said. “So far, we’re not yet to the level that we were at last year, which was really our worst year since pre-Covid... but it’s certainly going up.” He added: “Time will tell how bad it will get. Now is the time to be vigilant. I expect things to get worse before they get better.”

Across the United States, wastewater surveillance shows a country-wide pattern of rising activity, with some regions reporting higher-than-average levels. The West Coast has reported higher concentrations, while the South, Northeast and Midwest regions show more moderate signals that are trending upward. Public health experts emphasize that wastewater data, while not a direct count of cases, can help track overall trends, especially when many people do not undergo testing.

Beyond the community at large, the surge has touched local schools. In Massachusetts, Robert Elementary School outside Boston closed for two days last week after about 130 of roughly 600 students were absent due to suspected norovirus illness. In New Jersey, Central Elementary in Haddonfield shuttered for two days after an “unusual number” of students reported symptoms, with deep-cleaning measures implemented before students and staff could return ahead of the holidays. Officials said such closures aim to curb transmission while maintaining as much instructional time as possible.

Travel-related exposure has also contributed to the broader picture. More than 100 people have fallen ill aboard the AIDAdiva, a German cruise ship conducting a 133-day world itinerary with stops in multiple continents, including England, Canada, the United States, Mexico, Panama and Japan. Cruise ships can be hotspots for norovirus due to close quarters and shared facilities, though health authorities note that rapid reporting and cleaning protocols help limit wider outbreaks.

Experts say GII.17 emerged in Romania in 2021 after genetic re-assortment events among circulating strains and quickly established dominance in several regions before becoming the predominant driver of cases in the United States during the last virus season. Data for the 2024–2025 outbreak season show the variant behind at least 75 percent of outbreaks each month, underscoring how rapidly it has become established in the country’s transmission network.

The current season’s pattern comes as overall flu activity remains low nationwide but is rising in most regions. Public health officials caution that simultaneous activity across respiratory and gastroenteric infections can complicate clinical management and strain healthcare systems during peak travel and holiday periods. In South Carolina, measles outbreaks have emerged with 123 cases reported this year and 27 since Friday, December 5, while New York, Louisiana and Colorado report higher influenza activity. Internationally, several countries, including the United Kingdom, are facing a notably severe flu season after earlier-than-usual spikes.

Public health officials emphasize practical precautions to limit norovirus transmission. Hand hygiene, careful cleaning of contaminated surfaces, and staying home when sick are crucial steps to prevent spread. Given the absence of a vaccine for norovirus, clinicians encourage prompt hydration and, in vulnerable populations, medical attention for dehydration or persistent symptoms. While the landscape of norovirus may be shifting with the GII.17 strain, health authorities note that standard prevention efforts remain the frontline defense for communities and institutions facing ongoing outbreaks.