Doctors are warning people to stay watchful as human metapneumovirus (HMPV), a seasonal respiratory virus connected to illnesses such as pneumonia and bronchitis, is circulating widely in California.
Recent data from the WastewaterSCAN Dashboard shows increased levels of HMPV detected in wastewater samples across dozens of Northern California cities. Among the areas reporting the highest levels is Redwood City, with additional increases seen throughout the San Francisco Bay Area and parts of Napa Valley.
Health experts say the timing is not unusual.
“The reason we are seeing this right now is that late fall and winter are always the seasons for respiratory pathogens, mainly because viruses like cold conditions, and because of people gathering indoors without ventilation. This season, unlike the past four winters, we did not have high rates of COVID-19, allowing other viruses to flourish,” said Monica Gandhi, a professor of medicine and infectious disease specialist at the University of California, San Francisco.
Wastewater monitoring shows that HMPV levels are currently highest in California and along parts of the Northeast. Nationally, reported cases have been increasing since October 2025, according to data from the Centers for Disease Control and Prevention (CDC). The virus typically spreads during winter and early spring.
“A high COVID-19 or influenza season can lead to broad immunity for a short period of time, a phenomenon called ‘viral interference,’ where viruses that cause the common cold cannot compete,” Gandhi said. “However, this season, given the lower rates of COVID-19, we had high rates of influenza in the early winter and are now seeing a virus which causes cold-like symptoms: HMPV.”
For most people, an HMPV infection causes symptoms similar to a typical cold, including cough, congestion, and a mild fever, which usually last less than a week. However, in some cases, the virus can lead to more serious complications such as ear infections, severe asthma attacks, or a distinctive barking cough.
According to the Mayo Clinic, the virus may also lead to illnesses like croup or pneumonia and can sometimes cause secondary bacterial lung infections. While most patients recover quickly, those with weakened immune systems or respiratory conditions may experience more severe symptoms.
In rare cases, infection can become life-threatening for vulnerable groups such as infants, especially between 6 and 12 months old, the elderly, and those with compromised immune systems.
“For those individuals, I would recommend not going to large gatherings or being inside in unventilated spaces right now,” Gandhi said.
Although the virus has drawn attention in public health data, it may be less noticeable in high school environments.
“This age in general is healthy, and their immune system has been tested for a few years and has memory T cells to fight off viruses,” said Jennie Liebermann, a Sequoia Union High School District nurse.
Still, respiratory viruses are a common concern for school health offices. The district decides when to send illness or health notifications to families by following guidance from the CDC and the California Department of Public Health (CDPH), which forms thresholds for when schools should warn communities about disease activity.
Liebermann sees the role of school health services changing over the next decade, saying that there will be an increase in the acuity of students seen in the health office.
“As health care becomes more expensive, families will use the health services that are available to them, such as the school health office,” Liebermann said.
Unlike influenza or RSV, there are currently no vaccines available to prevent HMPV infection. Diagnosis usually needs laboratory testing, but many mild cases go undetected because symptoms resemble those of a common cold.
“In gatherings, ensure there is ventilation in the room, which is one of our best non-pharmaceutical interventions to prevent the spread of respiratory pathogens,” Gandhi said.
Basic preventive habits remain one of the most effective ways to reduce transmission in schools and communities.
“There’s a rule that a lot of students don’t follow. It states that if you have a fever, you should stay home for 24 hours until it breaks without medication. Some students can have a fever in the morning, take some time, and come to school,” said Erica Quintana, the health aide at Carlmont. “Regardless of whether your fever breaks with the medication, you’re most contagious when you have a fever. So if you ever have a fever, you need to stay home for 24 hours, frequently wash your hands, and cough into your arm.”
Quintana also recommends wearing a new mask each day, as changing masks often is important.
“Do not share items, even makeup, because you can spread germs through makeup as well. So you just have to keep yourself clean frequently,” Quintana said.
Medical experts say that most people will encounter HMPV at some point in their lives. According to the Cleveland Clinic, almost everyone contracts at least one strain of the virus during childhood, often before age 5.
However, Gandhi says the virus usually does not lead to major spikes in severe illness.
“MPV generally causes the common cold or an upper respiratory tract infections so patients don’t always present to the clinic or hospital with such symptoms. We are seeing young children and older people come to the clinic with symptoms, but are not seeing an uptick in hospitalizations due to HMPV above what we see in the usual respiratory pathogen season,” Gandhi said.
