In a statement on Wednesday, Denmark's State Serum Institute (SSI) warned of a spike in mycoplasma pneumoniae, a bacteria that causes inflammation of the lungs.
China’s health ministry last week said that a surge in cases of mycoplasma pneumoniae, flu and RSV was behind a dramatic rise in the number of children being admitted to hospital. Immunity among the population remains low after several harsh Covid lockdowns.
An alert sent through ProMed – an international database monitoring disease outbreaks – had initially warned of children presenting to hospitals in Beijing and Liaoning with a “pneumonia like” illness, prompting concern that a new pathogen was spreading throughout the country.
The SSI said that infections of mycoplasma pneumoniae in Denmark were high enough to be classed as an epidemic.
A total of 541 cases were reported last week, more than triple the total number recorded in October.
The SSI's senior researcher Hanne-Dorthe Emborg said that there are “significantly more cases than normal” and that prevalence was increasing across the country.
“Mycoplasma infections come in waves, with the disease affecting groups in the population who have not yet built up immunity,” she said.
“That is why it is also typically a disease seen among school children aged 6-12.”
The country typically experiences outbreaks of mycoplasma pneumoniae every four years.
“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,” she added.
“We have actually been waiting for it since we closed the country after the Covid pandemic.”
Symptoms include sore throat, fatigue and a slowly worsening cough that can last for weeks or months.
The disease is usually treated with a five-day oral course of azithromycin.
The Netherlands last week said that it was experiencing a surge in pneumonia cases, but did not specify whether mycoplasma pneumoniae was the source.
There were 103 pneumonia infections for every 100,000 children aged five to 14 in the week up to November 19, according to data published by the Netherlands Institute for Health Services Research (NIVEL).
This was a rise of 24 per cent from the 83 reported the week before, the data showed.
Scientific experts have stressed that it is unlikely that a new pathogen is behind the surge in cases in China, and that the risk of a new pandemic remains low.
But the UK Health Security Agency has said that it is “monitoring” the situation in China and keeping an “open mind” about the cause of the wave of respiratory illness.
In December last year, the UK saw a huge spike in cases of Strep A among young children - in part driven by a lack of immunity caused by previous Covid-19 restrictions. The UK also suffered a particularly bad flu season.