Denmark and the Netherlands are the latest countries to report an outbreak of pneumonia in children, days after China.

A post on Avian Flu Diary, an infectious disease news blog, revealed Mycoplasma pneumoniae infections have reached the epidemic level, with an increase that began in the summer but has risen significantly over the past five weeks.

“The number is now so high that it is an epidemic,” according to Denmark's Statens Serum Institut.

"In the past five weeks, the number of new cases has increased significantly, and we are now seeing significantly more cases than usual, and there is widespread infection throughout the country," senior researcher Hanne-Dorthe Emborg from the Statens Serum Institut was quoted as saying.

In week 47, 541 new cases of Mycoplasma pneumoniae infection were detected, which is more than threefold since week 42, when the number of detected cases was 168.

The real number of cases is probably much higher, as not everyone with mild symptoms is tested.

However, Emborg said the cases are “not unusual” to Denmark, which historically sees nationwide epidemics of Mycoplasma pneumoniae infections approximately every four years.

She said the incidence is typically highest in autumn and early winter.

"For the past four years, the number of mycoplasma infections has been extremely low, and it is therefore not unusual that we have an epidemic now. We have been waiting for it since we closed the country after the COVID-19 pandemic," Emborg said.

The disease often presents as mild flu-like symptoms with fatigue, headache, sore throat and prolonged dry cough, especially nocturnal cough.

The vast majority have a fever, but often not as high a fever as with influenza and other more classic pneumonia.

This has given it the nicknames 'cold pneumonia' or 'atypical pneumonia', as regular penicillin has no effect on the infection either, the post said.

Last week, the Netherlands reported a significant uptick in pneumonia cases in children and young people since August, as per a government surveillance report on FluTrackers, an infectious disease news message board.

According to the Netherlands Institute for Health Services Research (NIVEL) last week, 103 of every 100,000 children between ages 5 and 14 suffered a bout of pneumonia.

This was up 24 per cent from the 83 recorded in the seven days before, data from NIVEL showed.

This is reportedly the largest outbreak of pneumonia NIVEL has recorded in recent years.

In 2022, the weekly average during peak flu season saw 58 kids fall ill with pneumonia. Cases in under-fours were also up from 124 to 145 per 100,000 in the same period.

Local media from the Netherlands reported that neither NIVEL nor the National Institute for Public Health and the Environment are able to explain the reason behind the increasing pneumonia cases.

Meanwhile, a new report in the Lancet shows a resurgence of Mycoplasma pneumonia cases post  COVID in several countries including Singapore and Sweden since April.

The rising pneumonia cases in these countries come against the backdrop of reports of paediatric hospitals and clinics in China being overwhelmed with Mycoplasma pneumonia cases.

The surge in respiratory infections in China raised fears that a novel pathogen was behind the rise, which the country refuted.

Chinese officials informed the World Health Organisation that no new pathogens were detected in the outbreak, and instead, the illnesses were caused by known seasonal viruses such as the flu and RSV, along with the bacteria Mycoplasma pneumoniae.