Health authorities globally are on alert as China experiences a spike in pneumonia among children. The World Health Organisation has been monitoring a rise in respiratory illness in children living in northern China since mid-October. On November 23, on request from WHO, Chinese authorities shared data showing an increase in Mycoplasma pneumoniae cases in children since May and RSV, adenovirus and influenza since October. WHO reported that while the rise in some illness was happening earlier than usual, it was "not unexpected given the lifting of COVID-19 restrictions" as seen similarly in other countries. But Deakin University chair in epidemiology Catherine Bennett said it was the number of paediatric pneumonia cases that was concerning health authorities. "The main concern is if the rise in childhood pneumonia were to indicate a new pathogen, or new levels of disease severity," she said. "So far we have not heard reports of either, but it is important that the source of infection can be confirmed to rule out these concerns, and any possible implications outside of China." Local media FTV News reported in Beijing and Liaoning, in the country's north-east, children's hospitals were "overwhelmed with sick children". UNSW master of infectious diseases intelligence program director Dr Abrar Chughtai said rapid action would be required if the rise in cases was not due to existing pathogens. "Although Chinese authorities did not detect any unusual or novel pathogens, I am concerned about reports of 'undiagnosed pneumonia'," he said. "Previous major respiratory epidemics/pandemics also started like this. "So at this stage we don't need to panic, but enhanced surveillance and testing is needed." It is the first winter China is experiencing since fully reopening its international borders after the COVID-19 pandemic. Ms Bennett said the country was experiencing uplifts in infectious diseases similar to other countries including Australia since opening up. "Young children in school in China will have spent up to half their life without the usual exposure to common pathogens, and so do not have the same levels of immunity," she said. "A drop in immunity against common pathogens can lead to both more infections, and more severe disease when an infection occurs. "The combination of both can lead to substantial rises in the number of people in hospital with infections." WHO has advised against any travel or trade restrictions on China and said it was continuing to monitor the situation "in close contact with national authorities".