What to know about new norovirus strain after major mutation

As autumn arrives in the Bay Area, so does the season for colds and flu—along with an increase in norovirus cases, a highly contagious bug known for causing vomiting and diarrhea.

Often called the stomach flu, norovirus typically spreads across North America from November to April. While generally less dangerous than influenza or COVID-19, it can lead to severe dehydration, especially in infants and older adults. Most people experience unpleasant stomach issues for a few days and then recover.

That said, the reality is a bit more complicated. public health experts suggest that this year, the virus might spread more rapidly due to a recently emerged strain. According to the Centers for Disease Control and Prevention (CDC), this new strain is more contagious and was responsible for increased outbreaks last year. Peter Chin-Hong, a UCSF professor of microbiology and immunology, believes it is likely contributing to the high transmission rates observed in certain areas of the Bay Area.

Here's what you should know about this stomach bug as the holidays approach, including how to protect yourself:

**What makes the new strain different?**

According to Chin-Hong, norovirus had remained relatively stable for about 30 years before last year's major mutation. This meant our immune systems were better equipped to recognize and fight off the virus. That said, the reality is a bit more complicated. last year saw the emergence of GII.17, which quickly became the dominant strain in the U.S. The CDC reports that this strain is more contagious and caused a surge in norovirus outbreaks nationwide last winter. CDC researchers believe it originated from an outbreak in Romania in 2021.

In Santa Clara County, officials reported two norovirus outbreaks between autumn 2023 and spring 2024. As the new variant spread during the 2024-2025 season, the county saw a significant increase to 16 outbreaks, according to a public health spokesperson.

**How widespread is the virus in the Bay Area?**

Wastewater testing indicates elevated levels of the virus in Silicon Valley, San Francisco, and Vallejo. Despite this, public health officials are not yet issuing any major warnings.

Chin-Hong notes that the presence of the virus in wastewater usually precedes actual cases and hospitalizations. A Santa Clara County spokesperson stated that there have been no norovirus outbreaks reported to the public health department in the last three months. Wastewater data also suggests that the virus is currently less prevalent than it was during its peak last winter.

A Contra Costa County spokesperson said that transmission rates “appear consistent with what we typically see this time of year, based on a low number of reported outbreaks and current lab surveillance data.” While county health experts cannot predict the precise impact of the new strain, they acknowledged that “it has been associated with more severe illness.”

Chin-Hong anticipates increased spread of the virus this month due to ongoing holiday gatherings.

**How can I prevent norovirus?**

Unlike COVID-19 and the flu, which generally require exposure to a larger number of viral particles to cause infection, norovirus can cause illness with as few as five to ten particles. Dr. Dean Winslow, a Professor of Medicine and Pediatrics at Stanford, notes that transmission often increases during the holidays as people gather in homes. “These outbreaks of winter vomiting illnesses are nothing new,” Winslow stated.

Norovirus spreads when microscopic particles from an infected person's feces or vomit enter another person's mouth. This often happens through contaminated hands, surfaces, food, or water. Frequent hand washing is a key preventative measure, but it's important to note that hand sanitizer is not very effective against norovirus.

The CDC recommends practicing safe food handling and preparation, including thoroughly washing produce, sanitizing cookware, and regularly disinfecting household surfaces. If you suspect that clothing may be contaminated with norovirus particles, wash them in hot water.

**What should I do if I get norovirus?**

If you become ill, Dr. Winslow advises staying home, washing your hands frequently, and disinfecting surfaces, particularly in bathrooms where vomiting is likely to occur. He also recommends drinking fluids with electrolytes to replenish minerals lost through vomiting and diarrhea.

“Staying hydrated is the most important thing,” he emphasized.

Fortunately, Dr. Winslow says the illness is usually short-lived, with symptoms typically lasting one to three days and peaking early on.