Cases of acute hepatitis of unknown origin raise speculation that these could be triggered by a new strain of adenovirus or even Covid-19.
Eleven countries in the World Health Organization European Region and the US have reported at least 169 cases of acute hepatitis of unknown origin in children and young people.
Cases have been reported in the UK (114), Spain (13), Israel (12), the US (9), Denmark (6), Ireland (5), the Netherlands (4), Italy (4), Norway (2), France (2), Romania (1), and Belgium (1), a WHO statement said on Saturday.
The hepatitis, an inflation of the liver, had raised speculation in the medical arena that the cases could be triggered by a new strain of adenovirus or even Covid-19.
Covid-19 (SARS-CoV-2) has been reported in 20 of the cases, and 19 were detected with the coronavirus and adenovirus co-infection, said the world health group.
"While adenovirus is currently one hypothesis as the underlying cause, it does not fully explain the severity of the clinical picture," the WHO said.
Those with cases are aged 1 month to 16 years, and 17 children (approximately 10 percent) have needed liver transplantation, and at least one death has been reported.
Britain first reported an unexpected significant increase in cases of severe acute hepatitis of unknown origin in young, generally previously healthy children.
Many cases reported gastrointestinal symptoms, including abdominal pain, diarrhoea, and vomiting preceding presentation with severe acute hepatitis.
The common viruses that cause acute viral hepatitis (hepatitis viruses A, B, C, D, and E) have not been detected in any of these cases.
Based on the currently available information, international travel or links to other countries have not been identified as factors.