Surge of Mysterious Respiratory Illness Hits US, With No Vaccine or Cure

Human metapneumovirus (HMPV) cases spike across the country, especially in Northern California and the Northeast.

Mar. 25, 2026 at 3:35am by Ben Kaplan

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The US is experiencing a dramatic surge in human metapneumovirus (HMPV), a little-known respiratory illness that currently has no vaccine or antiviral treatment. The surge first began in Northern California in mid-February and has since spread to other parts of the country, including the Northeast. HMPV presents with symptoms similar to other respiratory illnesses like COVID-19, the flu, and RSV, making it difficult to identify. While mild cases may resolve within a week, severe infections can lead to pneumonia, especially in vulnerable populations like young children and the elderly.

Why it matters

The lack of a vaccine or antiviral treatment for HMPV is concerning, as the illness can be life-threatening for certain groups. The surge in cases across the US highlights the need for better surveillance, testing, and treatment options for this emerging respiratory threat.

The details

HMPV is not a reportable disease in California, but the state's Department of Public Health is tracking the surge through voluntary data from clinical labs and wastewater surveillance. Compared to the last five seasons, HMPV activity is higher than all prior seasons except 2022-2023. HMPV is difficult to identify because it presents with symptoms similar to other respiratory illnesses. A separate test is usually required in the emergency room to diagnose HMPV, as it is not included in the standard nasal swab that checks for COVID-19, the flu, and RSV. Severe HMPV infections can lead to pneumonia, especially in those with underlying conditions like asthma or in smokers.

  • The surge in HMPV cases first began in Northern California in mid-February 2026.
  • HMPV typically circulates in late winter and early spring.

The players

Kristine Tuzon

A spokeswoman for the California Department of Public Health.

Dr. William Schaffner

A professor of preventive medicine and infectious diseases at the Vanderbilt University School of Medicine.

Dr. Peter Chin-Hong

An infectious diseases specialist at the University of California, San Francisco.

Dr. Manisha Newaskar

A pediatric pulmonologist at Stanford Medicine's Children's Health Division.

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What they’re saying

“Virtually every child that is born and raised in the US — and indeed around the world — by the time they get to age five, they will have experienced infection with human metapneumovirus. And like RSV and like influenza, you can get infected again and again and again.”

— Dr. William Schaffner, Professor of preventive medicine and infectious diseases

“We typically don't test for HMPV unless someone is critically ill or at least hospitalized. We test when we are trying to find out the etiology of a pneumonia, especially in the ICU where it may matter most.”

— Dr. Peter Chin-Hong, Infectious diseases specialist

“Every week, I see at least 1 or 2 severe cases, who have needed support therapy, such as oxygen, IV fluids, or ventilators.”

— Dr. Manisha Newaskar, Pediatric pulmonologist

What’s next

Pharmaceutical companies are developing HMPV vaccines, which could be available in as little as 5 years. A combination vaccine protecting against COVID-19, the flu, and HMPV in a single shot is also in development.

The takeaway

The surge in HMPV cases across the US highlights the need for better surveillance, testing, and treatment options for this emerging respiratory threat, especially as no vaccine or antiviral treatment currently exists. Protecting vulnerable populations, such as young children and the elderly, will be crucial as this illness continues to spread.