What to know about the respiratory illnesses surging in some parts of China

A surge of respiratory illnesses related to flu, respiratory syncytial virus (RSV) and other infections has been sweeping across some parts of China and mainly infecting children, as the country experiences its first full winter since easing COVID-19 restrictions.

Amid media reports of "undiagnosed pneumonia" cases overwhelming hospitals over the past few weeks, health authorities have insisted that the uptick is linked to common illnesses.

Since May, China has been experiencing an increased number of children sick with mycoplasma pneumoniae – bacteria that causes mild infections of the respiratory system – as well as pediatric cases of RSV, adenovirus, influenza and COVID-19 since the fall, according to the World Health Organization (WHO).

Last week, the WHO submitted an official request to China for information on "an increase in respiratory illnesses and reported clusters of pneumonia in children."

No new viruses or unusual conditions have been identified, according to Mi Feng, spokesman for China's National Health Commission, who spoke at a press conference on Sunday.

"According to the analysis, acute respiratory diseases in China have continued to rise recently, which is related to the superposition of multiple respiratory pathogens," he said.

Mi Feng said that with the coming of winter and the increased number of respiratory illnesses, China's National Health Commission and the National Administration of Disease Control and Prevention are continuing to monitor these diseases, promote vaccination, dispatch medical resources, and make sure people receive treatment.

Following a meeting with local health authorities, the WHO noted that while the increase was earlier in the season than unusual, it was "not unexpected given the lifting of COVID-19 restrictions, as similarly experienced in other countries."

Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor, agreed with the WHO assessment that the surge is likely due to the lifting of COVID-19 restrictions. "These known pathogens are causing a rise in cases, particularly among children, but there have been no reports of unusual or novel viruses, and hospital capacities remain manageable," he said.

When COVID-19 began spreading across the world in January 2020, China initiated some of the harshest restrictions anywhere in an attempt to prevent outbreaks. Known as the "zero COVID" policies, they included lockdowns and mass testing. The country officially lifted its zero COVID policies in January 2023, and ended the last of its primary pandemic-era measures this past summer.

Brownstein said a seasonal increase in respiratory illnesses occurring as COVID-19 restrictions are being lifted is not exclusive to China, but is something seen globally. When the U.S. lifted many restrictions last year it experienced a very similar pattern, with most states reporting high levels of flu-like activity.

"The co-circulation of respiratory viruses and bacteria during the colder months often places additional burden on healthcare facilities, making it crucial for public health measures, including vaccination and enhanced surveillance, to be in place to manage these expected spikes in respiratory cases," Brownstein said.

Photos and videos have shown overcrowded areas at hospitals in China, including Beijing Children's Hospital. Reported cases among children are especially high in northern areas like Beijing and Liaoning province, where hospitals are warning of long waits to be seen.

Mi Feng said hospitals in densely populated areas with long wait times can lead to a risk of cross-infection, and urged parents to instead take their children, if they have mild symptoms, to grassroots medical and health institutions or general hospitals for pediatric care.

He also advised people to follow known and proven mitigation measures, including wearing masks, increased ventilation, and frequent hand washing.

On Sunday, China's health ministry also urged local authorities to increase the number of fever clinics.

ABC News' Karson Yiu contributed to this report.