As one of the first countries in Europe, Scottish authorities alerted about an observed unusual number of severe liver inflammation (hepatitis) in children in April 2022. The infection could not be linked to commonly known causes for hepatitis. Until mid-June 2022, 427 cases of paediatric hepatitis of unknown origin were notified from 20 countries across the World Health Organization’s European Region for the period 1 January to 16 June 2022.
In their outbreak report published in Eurosurveillance, Vidal et al.  analyse demographic, epidemiological, clinical and microbiological data available from The European Surveillance System at the European Centre for Disease Prevention and Control.
Most affected: children under five years
Most of the severe infections were diagnosed among very young children between zero and five years of age (77% or 330) and every second child (53%) tested positive for adenovirus. In the United Kingdom (UK), the majority (68%) of children with acute hepatitis of unknown origin were diagnosed with adenovirus at the same time.
A distinctly smaller fraction among the notified 427 children had SARS-CoV-2 (10%) and the same number of children was coinfected with both adenovirus and SARS-CoV-2.
The highest proportion (60%) of positive tests for adenovirus was found among the youngest group of children aged 0 to five years. At 36%, this group also accounted for the highest number of admissions to intensive care or high-dependency units (ICU/HDU) due to their acute liver inflammation compared with the age group of six- to 10-year-olds (22%) and adolescents between 11 and 16 years of age (10%).
In addition, 16 of the 18 children across the WHO European Region who needed a liver transplant were younger than five years old (but the authors did not find a significant difference by age group in the odds of transplantation).
Analysing the data, Vidal et al. found that those children with adenovirus infections were significantly more likely to be admitted to ICU/HDU due to their severe hepatitis and “adenovirus infection is therefore one of the main aetiological hypotheses under investigation, although a causal relationship has not been proven and additional cofactors are under investigation.”
Observed correlation between adenovirus positivity and disease severity
More than half of the cases (59%) occurred between weeks 9 and 17 of 2022 with an observed decline from week 18 onwards. The authors however point out at a possible reporting delay caused by the fact that severe hepatitis might only develop weeks after the onset of the first symptoms and thorough investigations to exclude specific causes could take several days or even months which would make recent trends challenging to interpret.
Looking at data from reported cases with a result on testing for adenovirus across different European countries, prevalence of adenovirus infection was 64% among the children in the UK compared with 35% in other countries (p < 0.001). When adjusting by age, children with severe hepatitis in the UK were still significantly more likely to have an adenovirus infection than cases in other countries (odds ratio = 3.10; 95% confidence interval: 1.90–5.07).
More than half of the children (53%, 74/140) with severe hepatitis in the UK were admitted to ICU/HDU compared with other European countries where 8 of the 101 children were hospitalised (p < 0.001).
Vidal et al. summarise that their findings “may indicate a higher likelihood of severe disease in younger children, but what is not clear is whether reported cases across all ages share the same aetiology. It may also prove difficult to identify a definite causative agent”.
They call for greater efforts to establish whether the reported cases are above the baseline incidence of severe hepatitis of unknown aetiology in children and to better define this syndrome and standardise the diagnostic algorithms.
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References/notes to editors:
 Adriana Romaní Vidal, Aisling Vaughan, Francesco Innocenti, Soledad Colombe, Lina Nerlander, Natalia Rachwal, Bruno Christian Ciancio, Aikaterini Mougkou, Carlos Carvalho, Enrique Delgado, Pers Mook, Géraldine de Muylder, Michael Peeters, Tencho Tenev, Elitsa Golkocheva-Markova, Veronika Vorobieva Solholm Jensen, Anders Koch, Julie Figoni, Cécile Brouard, Georgia Nikolopoulou, Anastasia Zisouli, Niamh Murphy, Annemarie Broderick, Lital Goldberg, Rivka Rich, Lior Hecht Sagie, Maria Elena Tosti, Barbara Suligoi, Rosa Joosten, Roan Pijnacker, Ingvild Fjeldheim, Eli Heen, Małgorzata Stępień, Piotr Polański, Rui Tato Marinho, João Vieira Martins, Carmen Varela, Ana Avellón, Emmi Andersson, Marie Jansson Mörk, Sema Mandal, Conall Watson, Laura Coughlan, Meera Chand, Claire Neill, Declan T Bradley, Kathy Li, Maureen O’Leary, Neil McInnes, Christopher J Williams, Catherine Moore, Ardiana Gijini, Erika Duffell, Richard Pebody
 Adenovirus can cause a wide range of illnesses, see e.g. https://www.cdc.gov/adenovirus/about/symptoms.html
 The WHO European Region comprises 53 countries, covering a geographical area from the Atlantic to the Pacific ocean. See: https://www.who.int/europe/about-us/about-who-europe/organization
Method of Research
Subject of Research
Hepatitis of unknown aetiology in children – epidemiological overview of cases reported in Europe, 1 January to 16 June 2022
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