Reports of an increase in the number of respiratory virus cases in China have evoked dark echoes of the start of the Covid-19 pandemic almost exactly five years ago.

But despite the superficial similarities, this situation is very different and far less worrisome, medical experts say.

Chinese cases have been reported to be infected with the human metapneumovirus, known to doctors as HMPV. Here’s what we know so far:

What is HMPV?

It is one of several pathogens that circulate around the world every year, causing respiratory diseases. HMPV is common — so common that most people will be infected when they are still children and may experience several infections during their lifetime. In countries with cold weather months, HMPV may have an annual season, similar to influenza, while in places closer to the equator it circulates at lower levels throughout the year.

HMPV is similar to a virus better known in the United States—respiratory syncytial virus, or RSV. It causes symptoms similar to those associated with the flu and Covid, including cough, fever, nasal congestion and wheezing.

Most HMPV infections are mild and resemble bouts of the common cold. But severe cases can result in bronchitis or pneumonia, especially in infants, the elderly and immunocompromised. Patients with pre-existing lung conditions, such as asthma, chronic obstructive pulmonary disease or emphysema, are at greater risk of severe outcomes.

In higher-income countries, the virus is rarely fatal; in lower-income countries with weak health systems and poor surveillance, deaths are more common.

How long has this virus been around?

The virus was identified in 2001, but researchers say it has been circulating among humans at least 60 years. Although it is not new, it does not have a name to recognize flu, Covid or even RSV, said Dr. Leigh Howard, associate professor of pediatric infectious diseases at Vanderbilt University Medical Center.

One reason is that the name is rarely mentioned, except when people are hospitalized with a confirmed case.

“The clinical features are really hard to distinguish from other viral diseases, and we don’t routinely test for HMPV like we do for Covid, influenza or RSV,” said Dr. Howard. “So most infections go unrecognized and attributed to anything respiratory.”

How does a person become infected with HMPV?

The virus is primarily spread by droplets or aerosols from coughing or sneezing, direct contact with an infected person or exposure to contaminated surfaces – basically the same way people get colds, flu and Covid.

Is there a vaccine? Or treatment?

There is no vaccine against HMPV. But there is a vaccine for RSV, and research is underway to find a vaccine that could protect against both viruses in one shot, since they are similar. There is no specific antiviral treatment for HMPV; treatment is aimed at managing symptoms.

What does China say about it?

Chinese authorities have acknowledged that cases of HMPV are increasing, but have stressed that the virus is a known entity and is not a major concern. The coronavirus that causes Covid-19 was a new pathogen, so the human immune system had not built up a defense against it.

At a press conference held by the Chinese Center for Disease Control and Prevention on December 27, Kan Biao, director of the Center’s Institute of Infectious Diseases, he said that cases of HMPV are on the rise among children 14 and younger. The increase is particularly visible in northern China, he said. The number of flu patients has also increased, he said.

Cases could spike during the Lunar New Year holiday in late January, when many people travel and gather in large groups, he said.

But all in all, said Mr. Khan, “judging by the current situation, the extent and intensity of the spread of respiratory infectious diseases will be lower this year than last year”.

Official Chinese data shows that HMPV cases have been on the rise since mid-December, both in outpatient and emergency settings, according to Xinhuastate news agency. Some parents and social media users were unaware of the virus and sought advice online, the media said; it called for calmness and common precautions such as frequent hand washing and avoiding crowded places.

ua routine press briefing on Fridaya Foreign Office spokesman reiterated that cases of flu and other respiratory viruses routinely increase at this time of year, but “appear to be less severe and spread on a smaller scale than last year”.

Chinese officials said last week that they would set up a monitoring system for pneumonia of unknown origin. This will include procedures for laboratories to report cases and for disease control and prevention agencies to verify and act on them, state broadcaster CCTV reported.

What was the response of the public in China?

On the Internet, amid comments from people who say they have never heard of HMPV and express concern that it is a new pathogen, state media is trying to reassure people, warning them not to blindly take antiviral drugs.

Some users joked that they could finally use up the masks they had accumulated during the coronavirus pandemic. Many commentators discussed the general rise in the disease, not just HMPV: “Why does the flu hurt so much” trended on Weibo, a social media platform, on Monday.

What does the World Health Organization say?

The WHO has not expressed concern. dr. Margaret Harris, a spokeswoman for the organization, cited weekly reports from Chinese authorities showing a predictable rise in cases.

“As expected for this time of year, the Northern Hemisphere winter, there is a month-to-month increase in acute respiratory infections, including seasonal influenza, RSV and human metapneumovirus,” she said by email.

Should I worry?

Reports coming out of China are reminiscent of those from the early, confusing days of the Covid pandemic, and the WHO is still urging China to share more information about the origins of the outbreak, five years later.

But the current situation is different in key respects. Covid was a virus that was transmitted to humans from animals and was previously unknown. HMPV has been well studied and there is a wide range of testing opportunities for it. Globally, there is widespread population immunity to this virus; it wasn’t, for Covid. A severe HMPV season can strain hospital capacity – especially pediatric wards – but not overburden medical centers.

“However, it is also vital for China to share its data on this outbreak in a timely manner,” said Dr. Sanjaya Senanayake, an infectious disease specialist and associate professor of medicine at the Australian National University. “This includes epidemiological data on who has become infected. Also, we will need genomic data to confirm that HMPV is the culprit and that there are no significant mutations to worry about.”

Vivian Wang contributed reporting from Beijing.