The dreaded norovirus — a “vomiting virus” that often causes stomach flu symptoms — is on the rise again in California, and doctors warn that a new subvariant could sicken more people this season.
In Los Angeles County, norovirus concentrations in wastewater have risen, indicating increased spread of the disease, the local Department of Public Health told the Los Angeles Times.
Norovirus levels are rising across California, with particularly significant increases in the San Francisco Bay Area and Los Angeles, according to the California Department of Public Health.
Norovirus testing and diagnosis rates are increasing across the country and in the western United States. The national test positivity rate was 11.69% in the week ending November 22, up from 8.66% two months ago. In the West, it’s even worse: up from 9.59% to 14.08%, according to the U.S. Centers for Disease Control and Prevention.
Norovirus is highly contagious and is the leading cause of vomiting and diarrhea in the United States, according to the Centers for Disease Control and Prevention. Outbreaks typically occur in the cooler months between November and April.
The situation is further complicated by the recent emergence of a new strain of norovirus, GII.17. The CDC says this development could lead to 50 percent more norovirus illnesses than typical.
“If your immune system doesn’t adapt to what’s going on around you, a lot of people are going to get infected,” said Dr. Peter Chin-Hong, an infectious disease expert at the University of California, San Francisco.
In the winter of 2024-25, GII.17 overturned the previously dominant norovirus strain GII.4, which was responsible for more than half of the country’s norovirus outbreaks over the past decade. According to CDC scientists, the ancestor of the GII.17 strain may have come from the subvariant that caused the outbreak in Romania in 2021.
GII.17 featured prominently during last winter’s norovirus surge, ultimately causing about 75% of the country’s outbreaks.
Writing in the journal Emerging Infectious Diseases, CDC scientists said the strain’s emergence coincided with a particularly bad year for norovirus, which started unusually early in October 2024, peaked earlier than normal in January and continued into the summer.
Chin-Hong said that during the first three seasons, when GII.4 was dominant, norovirus activity was relatively stable.
Norovirus can wreak havoc – as many parents know. An elementary school in Massachusetts was forced to cancel all classes Thursday and Friday because of a “high number of stomach bug cases” suspected to be caused by norovirus.
More than 130 students at Roberts Elementary School in Medford, Mass., were absent Wednesday, and administrators said classes may not resume on Friday with “a reasonable number of students and staff.” A company was hired to deep clean classrooms, door handles and kitchen equipment at the school.
However, some parts of California have not seen significant norovirus activity so far this season. The California Department of Public Health said that while norovirus levels in wastewater are increasing across the state, they remain at low levels.
So far this year, 32 laboratory-confirmed norovirus outbreaks have been reported to the California Department of Public Health. Last year, there were 69.
Officials caution that these numbers do not necessarily reflect the severity of norovirus in a given year because many outbreaks are not laboratory confirmed and because outbreaks can affect a small number of people or a large number of people.
According to the CDC, between August 1 and November 13, 153 norovirus outbreaks were publicly reported across the country. During the same period last year, there were 235 people.
UCLA has not reported an increase in the number of norovirus tests ordered, nor has there been a significant increase in test positivity rates. Chin-Hong said he hasn’t seen big growth at UCSF, either.
“The clinical situation in California is relatively stable, but I think it’s going to be a while before that comes,” Chin-Hong said.
Typically, norovirus causes 2.27 million outpatient visits, mostly among young children, 465,000 emergency department visits, 109,000 hospitalizations, and 900 deaths each year, mostly among people 65 and older.
People with severe persistent vomiting, severe diarrhea, and dehydration may need to seek medical attention for intravenous fluids.
“Dehydrated children may cry few or no tears and be unusually sleepy or irritable,” the CDC says. Sports drinks can help relieve mild dehydration, but what may be more helpful are oral rehydration solutions that can be purchased over the counter.
Experts say children younger than 5 years old and adults 85 and older are most likely to need to go to the emergency room or clinic due to norovirus and should not hesitate to seek care.
“Everyone is at risk, but the people you worry about, the people we see in the hospital, are the very young and the very old,” Jinhong said.
Those most at risk are babies, as it doesn’t take much to cause potentially serious problems. Newborns are at risk for necrotizing enterocolitis, a life-threatening intestinal inflammation that actually only affects newborns, according to the National Library of Medicine.
Healthy people usually clear the virus within one to three days, but immunocompromised people may continue to have diarrhea for a long time “because their body’s immune system cannot effectively neutralize the virus,” Chin-Hong said.
The main ways people can become infected with norovirus is by accidentally drinking water or food contaminated with feces, or by touching a contaminated surface and then putting their fingers in their mouth.
People usually develop symptoms within 12 to 48 hours after being exposed to the virus.
Experts say hand sanitizer is not effective against norovirus, which means proper handwashing is crucial.
The CDC says people should lather and scrub their hands with soap for at least 20 seconds, including on the backs of their hands, between fingers and under nails, then rinse and dry.
The CDC says a helpful way to keep track of time is to hum the “Happy Birthday” song from beginning to end twice. Chin-Hong said his favorite is the chorus of Kelly Clarkson’s “Since U Been Gone.”
If you live with someone who has norovirus, “if they touch surfaces and things, you really have to clean them,” Chin-Hong said. Contamination is very easy. Even small droplets of saliva on food that are breathed out and then eaten by someone else can spread the infection.
Throw away food that may be contaminated with norovirus, the CDC says. Norovirus is relatively heat-resistant and can survive temperatures as high as 145 degrees.
Norovirus is so contagious that even 10 virus particles are enough to cause infection. By comparison, it takes thousands of Salmonella particles to be ingested to get sick from the bacteria.
The CDC says people are most contagious when they are infected with norovirus, but they can still be contagious even after they feel better.
The CDC recommends staying home for 48 hours after infection. Some studies even show that “you can still spread norovirus two weeks or more after you feel better,” the CDC says.
The CDC also recommends washing clothes in hot water.
In addition to schools, other places where norovirus can spread quickly include cruise ships, day care centers and prisons, Chin-Hong said.
The most recent norovirus outbreak reported by the CDC occurred on the cruise ship AIDAdiva, which sailed from Germany on November 10. Of the 2,007 passengers on board, 4.8% reported illness. The outbreak was first reported on November 30 after stops in Portland, England; Halifax, Canada; Boston; New York City; Charleston, South Carolina; and Miami.
The ship is scheduled to call at Puerto Vallarta on Saturday, San Diego on Tuesday, Los Angeles on Wednesday, Santa Barbara on Thursday and San Francisco from December 19 to 21, CruiseMapper reported.
