LOS ANGELES, June 2 (Xinhua) -- As cases of COVID-19, flu and respiratory syncytial virus (RSV) were winding down across the United States, infections of another respiratory virus, called human metapneumovirus (HMPV), were picking up during spring and drawing concern as many people had not known about it.

Discovered in 2001, HMPV is in the Pneumoviridae family along with RSV. Broader use of molecular diagnostic testing has increased identification and awareness of HMPV as an important cause of upper and lower respiratory infection, according to the U.S. Centers for Disease Control and Prevention (CDC).

Cases of HMPV spiked in the United States this spring, according to the CDC's respiratory virus surveillance system. The percent of tests positive for HMPV surged to 17.5 percent for antigen tests and 9.6 percent for PCR tests at the end of March.

At its peak in mid-March, nearly 11 percent of tested specimens were positive for HMPV, a number that was about 36 percent higher than the average, pre-pandemic seasonal peak of 7 percent test positivity, according to a CNN report.

It filled hospital intensive care units with young children and seniors who are the most vulnerable to these infections.

HMPV can cause upper and lower respiratory disease in people of all ages, especially among young children, older adults, and people with weakened immune systems, according to the CDC.

Surveillance data from the CDC's the National Respiratory and Enteric Virus Surveillance System shows HMPV to be most active during late winter and spring in temperate climates.

Symptoms commonly associated with HMPV include cough, fever, nasal congestion, and shortness of breath, according to the CDC.

Unlike COVID-19 and the flu, there is no vaccine for HMPV or antiviral drugs to treat it. Instead, doctors care for seriously ill people by tending to their symptoms.

But there is no need to be too concerned about HMPV, a leading epidemiologist told Xinhua.

"The transmission mode, prevention methods, symptoms, and treatment of this virus are similar to influenza. So there is no need to worry too much except for the elderly people, the immunocompromised population and other high-risk groups," Zhang Zuofeng, professor and chair of the Department of Epidemiology at the University of California, Los Angeles, told Xinhua on Friday.