The reason, however, has remained “unknown and under investigation”, the WHO said in a statement.
Out of the 650 probable cases, at least 38 (6 per cent) children have required transplants, and nine (1 per cent) deaths have been reported to WHO. About one-third of the cases, 222, are in the UK, and another 216 have been reported in the US.
Besides, “99 additional cases are pending classification” the WHO said.
The global health agency added that the “cases are more clinically severe and a higher proportion develop acute liver failure compared with previous reports of acute hepatitis of unknown aetiology in children”.
It remains to be established whether and where the detected cases are above-expected baseline levels. WHO assesses the risk at the global level as moderate.
According to a joint surveillance report from the WHO and the European Centre for Disease Prevention and Control (ECDC), of a subset of 156 cases with data on hospital admission, about 14 per cent required intensive care, and 12 per cent needed a liver transplant.
About 75.4 per cent of cases were below 5 years of age.
While common viruses known to cause hepatitis, such as hepatitis A, B, C, D and E, have been ruled out in these kids, adenovirus — a common family of infections responsible for illnesses from colds to eye infections — has been suspected to be causing the condition.
Data from the UK showed adenovirus in 75 per cent of the cases, but the data for other countries are incomplete.
The WHO stated that adenovirus associated virus 2 (AAV-2) has also been detected in a small number of cases in the UK in liver and blood samples.
The strongest connection between the cases seems to be a virus called adenovirus 41, but adenovirus 41 is not an obvious culprit, experts say, because it has never been known to cause hepatitis in children who do not have weakened immune systems.
However, “further investigations are ongoing for the causative agent”, the WHO said.
The WHO also ruled out the hypotheses related to side effects from Covid-19 vaccines, stating that most of the affected children did not receive these vaccines.
Meanwhile, the UN health body suggested countries to “maintain regular hand washing and respiratory hygiene”.