A virus without a vaccine is spiking throughout California. Here’s what doctors say you need to know

Doctors are warning people to stay vigilant as a seasonal flu virus that can lead to pneumonia and bronchitis spreads in California wastewater and around the country.

High levels of human metapneumovirus – also known as HMPV – have been detected across dozens of Northern California cities, according to the public database WastewaterScan Dashboard.

The highest levels are reported in Redwood City, according to the dashboard. Elevated levels have also been found in the San Francisco Bay Area and Napa’s Wine Country.

In other parts of the country, HMPV levels are tracking lower. A map on the dashboard shows that HMPV levels are highest in California and along the Northeast coast.

Nationally, cases have been trending up since October 2025, according to data from the Centers for Disease Control and Prevention. HMPV usually circulates in the winter and spring, the agency notes.

“In the late winter, early spring, it can account for five percent to 10 percent of all the respiratory infections that we diagnose in the United States. So it’s definitely out there,” says Dr. Matthew Binnicker, director of the Clinical Virology Laboratory at Mayo Clinic, previously explained.

With a quieter Covid season, “other viruses like influenza and HMPV are getting their chance,” Dr. Monica Gandhi, an infectious disease specialist at UC San Francisco, told SFGate when asked about the state’s cases.

The viruses “compete with each other,” she said. “When we had big Covid surges, we saw really low rates of other common cold viruses.”

HMPV infection typically causes symptoms similar to a cold that last for only a few days to a week. But it may also result in more serious symptoms, like middle ear infections, life-threatening asthma attacks and a barking cough, experts caution.

“The major symptoms are cough, running nose, fever. It’s also prone to give you some wheezing, particularly if you have underlying asthma,” Dr. David Weber, a professor and Medical Director in Infectious Diseases at UNC Health, to WTVD in North Carolina.

In rare instances, infection can result in death in more vulnerable patients, including infants, the elderly and people who are immunocompromised, researchers say.

That’s also the case for other respiratory viruses spreading nationwide, such as flu and RSV.

Unlike the flu and RSV, there are no vaccines to prevent HMPV infections, which are diagnosed through laboratory testing.

Nearly everyone will catch at least one strain of HMPV during the course of their life, according to the Mayo Clinic. If you’re older than five, you’ve probably already had it at least once, the Cleveland Clinic notes.

Among U.S. adults, an estimated 12 hospitalizations occurred per 100,000 people each year from 2016-2019, a 2025 report showed.

Most people will recover with rest and fluids, and Weber says over-the-counter medications could ease symptoms.

A hospital may give patients an oxygen mask, steroids to reduce inflammation that leads to symptoms or fluid in an IV to help keep symptoms from worsening, according to the Cleveland Clinic.

The respiratory virus can spread through direct contact, sneezing, coughing or touching infected surfaces.

To prevent infection, the CDC recommends people regularly wash their hands with soap, clean surfaces and stay home when they are feeling unwell.

HMPV was discovered in 2001, but studies revealed that the virus had been circulating since the 1970s.