Norovirus activity is rising in parts of California, with wastewater and testing showing increases in Los Angeles County and the Bay Area. A newly dominant strain, GII.17, helped drive a large surge last winter and may increase cases because of lower population immunity. The CDC recommends strict handwashing (soap and water, 20 seconds), isolating sick people for at least 48 hours after symptoms end, and disinfecting surfaces to limit spread.
Norovirus Surge in California: New GII.17 Strain Raises Concern — What You Need To Know
Washing your hands is important to protect yourself from norovirus, which is resistant to hand sanitizer.(Ulrich Baumgarten via Getty Images)
The notorious norovirus — often called the “vomiting bug” — is rising again in California, and public-health officials are watching closely after a new subvariant, GII.17, became dominant and helped drive a large surge last winter.
What The Data Shows
Wastewater surveillance in Los Angeles County and the San Francisco Bay Area has detected increasing concentrations of norovirus, a strong indicator the virus is circulating more widely. Statewide trends reported by the California Department of Public Health show rising levels, though overall wastewater concentrations remain relatively low so far.
Testing data from the U.S. Centers for Disease Control and Prevention (CDC) also show higher test positivity rates: for the week ending Nov. 22, national positivity was 11.69% (up from 8.66% about two months earlier), while the Western U.S. was 14.08% (up from 9.59%).
Why GII.17 Matters
A new norovirus lineage, GII.17, supplanted the long-dominant GII.4 strain during the 2024–25 season. The CDC warns that when a novel strain spreads through a population with limited prior immunity, overall illness can increase by roughly 50% compared with typical seasons. GII.17 was associated with about 75% of U.S. outbreaks during last winter’s surge and is linked to an unusually early and intense season that began in October 2024 and extended into the summer.
How Norovirus Spreads and Who's At Risk
Norovirus spreads easily: through contaminated food or water, touching contaminated surfaces and then touching the mouth, and even via tiny droplets from someone’s mouth landing on food. The virus is extremely contagious — fewer than 10 viral particles can infect a person — and it is reasonably heat-resistant (can survive around 145°F / 63°C).
Symptoms typically start 12–48 hours after exposure and usually resolve in 1–3 days in healthy people. Infants, young children, adults 85 and older, and immunocompromised people face the greatest risk for severe illness, dehydration, hospitalization or complications. In newborns, rare but serious conditions such as necrotizing enterocolitis can occur.
Healthcare Impact
In a typical U.S. year, norovirus accounts for roughly 2.27 million outpatient visits, about 465,000 emergency-department visits, 109,000 hospitalizations, and around 900 deaths — mostly among seniors aged 65 and older. Severe ongoing vomiting, profuse diarrhea and dehydration may require medical attention and intravenous fluids.
Prevention and Practical Advice
Handwashing: Hand sanitizer is not reliably effective against norovirus. Wash hands thoroughly with soap and water for at least 20 seconds — include backs of hands, between fingers and under nails.
Isolation: Stay home while symptomatic and for at least 48 hours after symptoms end; some studies show people can shed virus for two weeks or more.
Cleaning: Disinfect high-touch surfaces frequently, wash laundry in hot water, and discard food that may have been contaminated. Because even tiny droplets on food can spread infection, err on the side of caution.
At-Risk Settings: Schools, day-care centers, cruise ships and correctional facilities are especially vulnerable to outbreaks. Recent cruise-ship reports named the AIDAdiva, with about 4.8% of passengers reporting illness during a late-November report.
When To Seek Care
Seek medical attention for signs of severe dehydration (very little or no urine, extreme thirst, dizziness, dry mouth, no tears in infants). For mild dehydration, oral rehydration solutions are preferable to sports drinks; children who are unusually sleepy or irritable should be evaluated promptly.
Bottom line: Norovirus activity is trending upward in parts of California and nationally. Vigilant handwashing, isolating sick people, careful cleaning, and quick attention to dehydration remain the best defenses.
