While COVID-19 and RSV swirled in the headlines in the past, a different virus with very similar symptoms silently dominated this spring — the human metapneumovirus (HMPV).

The U.S. Centers for Disease Control and Prevention reported near the end of May that cases of HMPV were far more prevalent this spring than those of other respiratory viruses.

Early-to-late March in particular saw a surge in the average number of HMPV tests being performed and found positive nationally.

In the week of March 4, over 19% of antigen tests and over 10% of PCR tests were flagged for HMPV — 36% higher than its seasonal peak before the pandemic.

Since patients aren’t usually tested for this virus outside of a hospital or ER, these figures likely underreport cases of HMPV, according to CNN.

For this reason, Dr. John Williams, a pediatrician at the University of Pittsburgh, told CNN that HMPV is “the most important virus you’ve never heard of.”

Like other respiratory illnesses, cases of HMPV can be severe and may lead to death among the immunocompromised, young children, and elderly adults, the CDC reported.

Being overweight or obese can heighten the risk of dying from HMPV-induced pneumonia, research suggests.

As The Dallas Express reported, susceptibility to COVID-19 was similarly linked to obesity due to a decreased bodily response seen toward fighting the illness.

Unlike COVID-19, no vaccine or antiviral drug treatment is thought to be effective against HMPV, meaning that those who suffer severe symptoms might need to be hospitalized.

Typical symptoms can include fever, cough, nasal congestion, and shortness of breath, and it is spread through close contact with an infected person that is coughing or sneezing.

Like typical respiratory infections, such as the common cold, the time between exposure to HMPV and the onset of symptoms is generally three to six days.

The arrival of warmer weather has already lowered the number of HMPV infections seen, as is typically the case with RSV and other respiratory illnesses.

Nonetheless, it bears noting that HMPV’s spike occurred just as cases of COVID-19 and RSV dropped.

“They would come in waves,” explained Dr. Pedro Piedra, a professor of molecular virology at Baylor College of Medicine, to Insider. “These viruses, whether they be influenza, or RSV, or human metapneumovirus, can have a significant consequence on our health.”