Cases of HMPV are on the rise in China - but what is it?

In recent weeks, scenes of hospitals in China overrun with masked people have made their rounds on social media, sparking worries of another pandemic.

Beijing has since acknowledged a surge in cases of the flu-like human metapneumovirus (HMPV), especially among children, and it attributed this to a seasonal spike.

But HMPV is not like Covid-19, public health experts have said, noting that the virus has been around for decades, with almost every child being 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 lead to a mild upper respiratory tract infection - practically indistinguishable from flu - for most people.

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

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

The very young, including children under two, are most vulnerable to the virus, along with those with weakened immune systems, including the elderly and those with advanced cancer, says Hsu Li Yang, an infectious diseases physician in Singapore.

If infected, a "small but significant proportion" among the immunocompromised will develop more severe disease where the lungs are affected, with wheezing, breathlessness and symptoms of croup.

"Many will require hospital care, with a smaller proportion at 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 the viruses survive better in the cold and they pass more easily from one person to another as people stay indoors more often.

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

In fact many countries in the northern hemisphere, including but not limited to China, are experiencing an increased prevalence of HMPV, said Jacqueline Stephens, an epidemiologist at Flinders University in Australia.

"While this is concerning, the increased prevalence is likely the normal seasonal increase seen in winter," she said.

Data from health authorities in the US and UK shows that these countries, too, have been experiencing a spike in HMPV cases since October last year.

Fears of a Covid-19 style pandemic are overblown, the experts said, noting that pandemics are typically caused by novel pathogens, which is not the case for HMPV.

HMPV is globally present and has been around for decades. This means people across the world have "some degree of existing immunity due to previous exposure", Dr Hsu said.

"Almost every child will have at least one infection with HMPV by their fifth birthday and we can expect to go onto to have multiple reinfections throughout life," says Paul Hunter, a medical professor at University of East Anglia in England.

"So overall, I don't think there is currently any signs of a more serious global issue."

Still, Dr Hsu advises standard general precautions such as wearing a mask in crowded places, avoiding crowds where possible if one is at higher risk of more severe illness from respiratory virus infections, practising good hand hygiene, and getting the flu vaccine.