Looking back on the outbreak of acute severe hepatitis with unknown etiology in children in 2022: What do we know now and where do we go from here?

An outbreak of acute severe hepatitis with unidentified etiology in children, which had been reported in 35 countries during March 2022 to September 2022, had aroused extensive public concern. The clinical manifestation of those pediatric patients was featured by elevated serum transaminases >500 IU/L, with exclusion of common A−E viral hepatitis. Previous reports had linked the unexplained hepatitis to a primary human adenovirus (HAdV) infection based on the high detection positivity of HAdV in patients' blood and stool samples, especially HAdV‐F41, while some other hypotheses centered around previous infections of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). However, the etiological link between the pediatric hepatitis and HAdV or coronavirus has not been established. Recently, three independent research groups have found compelling evidence supporting the pathogenesis of an unexpected adeno‐associated virus‐2 (AAV‐2) infection. AAV is a non‐enveloped, helper‐dependent, single‐stranded DNA virus from the Parvoviridae family. To date, at least 13 natural serotypes of AAV have been identified. Given the low immunogenicity and broad tissue tropism of AAV vectors, engineered AAV has been widely used as gene delivery vehicle for both proliferating and non‐proliferating cells. Liver is the main infection site of AAV and the link between liver‐associated diseases and AAV‐2 infection was reported earlier by introducing the underlying oncogenic effects of AAV‐2 integration, yet the conclusion remains controversial. Otherwise, few known diseases had been demonstrated to be associated with AAV infection, despite the high serological prevalence of AAV‐2 (30%−60%) and other AAV serotypes in adults. Therefore, although AAV‐2 had been detected in pediatric hepatitis patients with quantity far exceeding HAdV and SARS‐CoV‐2, using metagenomic sequencing according to earlier versions of the UK Health Security Agency reports, AAV‐2 infection has been scarcely suspected as a culprit until recently. Consistent with a widely recognized viral characteristic that AAV‐2 infection is not cytopathogenic, Morfopoulou et al. reported that no viral inclusions or direct viral cytopathic effect was observed in the pediatric patients' hepatocytes. Along with the observation of infiltrated activated T lymphocytes around AAV‐2 infected hepatocytes and the evidenced increase of T‐cell‐response‐associated cytokines, these data provisionally support a notion that immune‐ mediated pathogenesis constitutes the primary cause of liver injury in children. It is noteworthy that either HAdV, human herpesvirus 6 (HHV‐6), or Epstein−Barr virus was also tested positive in most cases, most probably as helper virus enabling AAV‐2 replication. Theoretically, assuming that helper viruses are absent, AAV‐2 per se could hardly induce liver damage severe enough to require liver transplantation, as input virions could only infect a very small portion of hepatocytes and therefore elicited limited immune responses. Nevertheless, the direct cytopathogenicity of AAV‐2 in hepatocytes as well as other tissue cells should be further evaluated, as another study by Ho et al. reported that viral inclusions existed in one certain patient's liver biopsy sample. Despite the high seroprevalence of AAV‐2 and other AAVs in human beings, the pathogenesis of host immune response against AAV infection is seldom reported, indicating either a low immunogenicity of AAV or a mildness of both innate and adaptive immunity elicited by the virus. Several plausible explanations for the contradiction between the severity of acute hepatitis in children and the viral characteristics of AAV‐2 might be, but not limited to: (1) Given the