In recent weeks, scenes of hospitals filled with masked people in China have been circulating on social media, raising concerns of another pandemic.

Beijing has since acknowledged that cases of flu-like human metapneumovirus (HMPV) have increased, especially in children, and attributed this to a seasonal spike.

But public health experts have said HMPV is not like COVID-19, noting that the virus has been around for decades, with almost every child becoming infected by their fifth birthday.

However, in some very young children and people with weakened immune systems, it can cause more serious illness. Here is what you need to know.

HMPV is a virus that will cause a mild upper respiratory tract infection for most people – practically indistinguishable from the flu.

First identified in the Netherlands in 2001, the virus spreads by direct contact between people or when someone touches surfaces contaminated with it.

Symptoms for most people include cough, fever and nasal congestion.

Su Lee Yang, an infectious disease doctor in Singapore, says very young people, including children under two years old, those with weakened immune systems, as well as the elderly and those with advanced cancer, are most vulnerable to the virus.

If infected, a “small but significant proportion” of immunocompromised people will develop more severe disease where the lungs are affected, causing wheezing, breathlessness and symptoms of croup.

“Many people will need hospital care, with very little risk of dying from the infection,” Dr Hsu said.

Like many respiratory infections, HMPV is most active during late winter and spring – some experts say this is because viruses survive better in the cold and they spread more easily from person to person. Because people often stay indoors.

In northern China, the current HMPV spike coincides with low temperatures that are expected to persist through March.

In fact, many countries in the Northern Hemisphere, including but not limited to China, are experiencing increased spread of hMPV, said Jacqueline Stephens, an epidemiologist at Flinders University in Australia.

“Although this is worrying, the increased prevalence is likely a normal seasonal increase seen in winter,” he said.

Data from health officials in the US and UK shows that these countries are also seeing a rise in HMPV cases since October last year.

Experts said fears of a COVID-19-style pandemic are too high, adding that pandemics are usually caused by new pathogens, which is not the case with HMPV.

HMPV is present globally and has been for decades. This means people around the world “have some degree of existing immunity because of past exposure,” Dr. Hsu said.

“Almost every child will have at least one infection with hMPV by their fifth birthday, and we can expect to have several re-infections throughout their life,” says Paul Hunter, a medical professor at the University of East Anglia in England.

“So overall, I don’t think there is any sign of a more serious global problem at present.”

Still, Dr. Hsu recommends standard general precautions such as wearing a mask in crowded places, avoiding crowds where possible if someone is at risk for more serious illness from a respiratory virus infection, and good hand hygiene. Adopt cleanliness and get flu vaccination.