Norovirus ‘vomiting bug’ rising in California. Here comes a new strain

The dreaded norovirus — the “vomiting virus” that often causes stomach flu symptoms — is on the rise again in California, and doctors are warning that a new subvariant could make even more people sick this season.

In Los Angeles County, concentrations of norovirus are already increasing in wastewater, indicating increased circulation of the disease, the local Department of Public Health told the Los Angeles Times.

Norovirus levels are increasing across California, and the increase is particularly notable in the San Francisco Bay Area and Los Angeles, according to the California Department of Public Health.

And the rate at which norovirus tests confirm infection is increasing nationally and in the western United States. For the week ending Nov. 22, the nationwide test positivity rate was 11.69%, up from 8.66% two months earlier. In the West, the situation is even worse: 14.08%, compared to 9.59%, according to the US Centers for Disease Control and Prevention.

Norovirus is extraordinarily contagious and is the leading cause of vomiting and diarrhea in America, according to the CDC. Outbreaks usually occur during the colder months, between November and April.

The picture is clouded by the recent emergence of a new strain of norovirus – GII.17. Such a development can lead to 50% more norovirus illnesses than normal, according to the CDC.

“If your immune system isn’t used to something that comes along, a lot of people get infected,” said Dr. Peter Chin-Hong, an infectious disease expert at UC San Francisco.

During the 2024-2025 winter season, GII.17 overthrew the previous dominant norovirus strain, GII.4, which had been responsible for more than half of national norovirus outbreaks over the previous decade. The ancestor of the GII.17 strain likely came from a subvariant that sparked an outbreak in Romania in 2021, according to CDC scientists.

GII.17 rose to prominence during last winter’s norovirus surge and was ultimately responsible for about 75% of the disease’s outbreaks nationwide.

The strain’s emergence coincided with a particularly bad year for noroviruses, one that began unusually early in October 2024, peaked earlier than normal the following January and extended into the summer, according to CDC scientists writing in the journal Emerging Infectious Diseases.

In the previous three seasons, when GII.4 was dominant, norovirus activity had been relatively stable, Chin-Hong said.

Norovirus can cause significant disruption – as many parents know all too well. A Massachusetts elementary school was forced to cancel all classes Thursday and Friday due to the “high volume of stomach illness cases” suspected to be caused by norovirus.

More than 130 students at Roberts Elementary School in Medford, Massachusetts, were absent Wednesday, and administrators said there likely would not be a “reasonable number of students and staff” to return to class Friday. A company was hired to do a deep cleaning of the school’s classrooms, doorknobs and kitchen equipment.

However, some areas of California are not seeing major norovirus activity so far this season. Statewide, although levels of norovirus in wastewater are increasing, they remain low, the California Department of Public Health said.

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 an outbreak can affect a small or large number of people.

Between Aug. 1 and Nov. 13, 153 norovirus outbreaks were publicly reported nationwide, according to the CDC. At the same time last year, there were 235.

UCLA has not reported an increase in the number of norovirus tests ordered, nor has it seen a significant increase in test positivity rates. Chin-Hong said he hasn’t seen a big increase at UC San Francisco either.

“Things are relatively stable clinically in California, but I think it will be a while before that happens here,” Chin-Hong said.

In a typical year, norovirus causes 2.27 million outpatient clinic visits, primarily by young children; 465,000 emergency room visits, 109,000 hospitalizations and 900 deaths, mainly among people aged 65 and over.

People experiencing persistent severe vomiting, deep diarrhea, and dehydration may need to see a doctor for intravenous hydration.

“Children who are dehydrated may cry with few or no tears and be unusually sleepy or fussy,” says the CDC. Sports drinks can help with mild dehydration, but oral rehydration fluids that can be purchased over the counter may be more helpful.

Children under 5 and adults 85 and older are more likely to need to go to the emergency room or clinic due to norovirus and should not hesitate to seek treatment, experts say.

“Everyone is at risk, but the people you worry about, the ones we see in the hospital, are the very young and the very old,” Chin-Hong said.

Babies are most at risk, because it doesn’t take much to cause potentially serious problems. Newborns are at risk of necrotizing enterocolitis, a potentially fatal inflammation of the intestine that affects almost only new babies, according to the National Library of Medicine.

While healthy people usually clear the virus within one to three days, immunocompromised people may continue to have diarrhea for a long time “because their body’s immune system cannot neutralize the virus as effectively,” Chin-Hong said.

The main way people get norovirus is by accidentally drinking water or eating food contaminated with feces, or touching a contaminated surface and then putting their fingers in their mouth.

People usually develop symptoms 12 to 48 hours after being exposed to the virus.

Hand sanitizer is not effective against norovirus — which means proper hand washing is essential, experts say.

People should lather their hands with soap and rub them for at least 20 seconds, including the backs of their hands, between their fingers and under their nails, before rinsing and drying them, the CDC says.

A helpful way to keep track of time is to hum the “Happy Birthday” song twice from start to finish, according to the CDC. Chin-Hong says his favorite is the chorus of Kelly Clarkson’s “Since U Been Gone.”

If you live with someone with norovirus, “you really need to clean surfaces and other objects if they touch them,” Chin-Hong said. Contamination is incredibly easy. Simply exhaling small droplets of saliva onto food that will later be consumed by someone else can spread an infection.

Throw away foods that may be contaminated with norovirus, says the CDC. Noroviruses are relatively heat resistant and can survive temperatures up to 145 degrees.

Norovirus is so contagious that 10 virus particles are enough to cause an infection. On the other hand, you have to ingest thousands of salmonella particles to get sick from this bacteria.

People are more contagious when they have norovirus — but they can still be contagious even after feeling better, according to the CDC.

The CDC advises staying home for 48 hours after infection. Some studies have even shown that “you can still spread norovirus for two weeks or more after you feel better,” according to the CDC.

The CDC also recommends washing clothes in hot water.

Besides schools, other places where norovirus can spread quickly are cruise ships, daycares and prisons, Chin-Hong said.

The most recent norovirus outbreak on a cruise ship reported by the CDC involved the ship AIDAdiva, which set sail on November 10 from Germany. Of 2,007 passengers on board, 4.8% reported illness. The outbreak was first reported on November 30 after shutdowns that month on the Isle of Portland, England; Halifax, Canada; Boston; New York City; Charleston, South Carolina; and Miami.

According to CruiseMapper, the ship was scheduled to call in Puerto Vallarta on Saturday, San Diego on Tuesday, Los Angeles on Wednesday, Santa Barbara on Thursday and San Francisco between December 19 and 21.