In an outbreak report on 23 April, the World Health Organization stated that 12 countries have reported at least 169 cases of hepatitis of unknown origin in children. The age range of cases is 1 month to 16 years old. Seventeen of the children have undergone liver transplants and at least one has died. None of the viruses that cause acute viral hepatitis (hepatitis viruses A, B, C, D and E) have been detected in any of the 169 cases.
The United Kingdom has had the greatest number of reported cases so far, with 108 cases as of 21 April according to the UK Health Security Agency. 77% of cases have tested positive for adenovirus. None of the confirmed cases is known to have received a COVID vaccine.
In the United States, the state of Alabama was the first to report cases. All nine of the reported cases have tested positive for adenovirus according to the U.S. CDC. Five of these were sequenced, and adenovirus type 41 infection was identified in all five. Adenovirus type 41 is known to commonly cause pediatric acute gastroenteritis, but is not known to be a cause of hepatitis in otherwise healthy children.
Currently the leading hypothesis for the causative agent involves adenovirus, with either an already-circulating variant severely impacting immunologically naive young children or there being a new variant with a more severe clinical profile. The former may be the result of less social mixing during the COVID pandemic. However, public health officials have emphasized that other hypotheses have not been ruled out. Other hypotheses include COVID infection or an unidentified toxin. Some have suggested COVID vaccination may be a cause and a recent paper has found that mRNA vaccine can enter a human liver cell line in vitro: "Our results indicate a fast up-take of BNT162b2 into human liver cell line Huh7, leading to changes in LINE-1 expression and distribution."
Will pediatric cases of hepatitis with unknown origin be conclusively linked to COVID vaccination?
This question will resolve on the basis of research findings/government reports. The systematic review (either peer-reviewed or a pre-print deemed credible by Metaculus) or statement by the WHO, US CDC, UK HSA, or EU ECDC that is most recent as of 1 April 2023 and which conclusively identifies the causative agent will be considered. If the causative agent is COVID vaccination, then this resolves positive.
If as of 1 April 2023 there is no systematic review or statement by the WHO, US CDC, UK HSA, or EU ECDC that identifies the causative agent, then Metaculus will conduct a review of all relevant published papers available as of 1 April 2023 that can be found via Google Scholar and that attempt to identify the causative agent. If at least 85% of these published papers claim a statistically significant and/or clinically meaningful association between severe pediatric hepatitis and COVID vaccination, then this will resolve positive.
COVID vaccination need not be the only cause of severe pediatric hepatitis, it just needs to be a main cause. e.g.: if cases of severe pediatric hepatitis are conclusively linked to some interaction between adenovirus infection and COVID vaccination, but not to either one of these on their own, then this will resolve positive.
Also see this question on whether pediatric cases of hepatitis with unknown origin will be conclusively linked to adenovirus infection.