Several experts pointed to winter's arrival, the end of Covid restrictions, and a lack of prior immunity in children as likely being behind the surging infections.

"Since China experienced a far longer and harsher lockdown than essentially any other country on Earth, it was anticipated that those 'lockdown exit' waves could be substantial in China," said Francois Balloux of University College London.

Unless there is new evidence suggesting otherwise, "there is no reason to suspect the emergence of a novel pathogen," he added.

Paul Hunter of the UK's University of East Anglia emphasised that "at present there is too little information to make a definitive diagnosis".

However "overall, this does not sound to me like an epidemic due to a novel virus," he added.

"If it was, I would expect to see many more infections in adults. The few infections reported in adults suggest existing immunity from a prior exposure."

Catherine Bennett of Australia's Deakin University pointed out that "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".