New York, March 31 : The mysterious hepatitis outbreak in US kids in 2022 is linked to a common childhood respiratory virus, studies have suggested. Paediatric hepatitis is rare, and doctors were alarmed when they started seeing outbreaks of severe unexplained hepatitis in otherwise healthy children after Covid-19 lockdowns eased in the US and 34 other countries in 2022. There have been about 1,000 cases to date; 50 of these children needed liver transplants and at least 22 have died. In a study, published in the journal Nature, researchers linked the disease to co-infections from multiple common viruses, in particular a strain of adeno-associated virus type 2 (AAV2). AAVs are not known to cause hepatitis on their own. They need "helper" viruses, such as adenoviruses that cause colds and flus, to replicate in the liver. Once they returned to school, children were more susceptible to infections with these common pathogens. The study led by a team of researchers from the University of California-San Francisco suggests that for a small subset of these children, getting more than one infection at the same time may have made them more vulnerable to severe hepatitis. "We were surprised by the fact that the infections we detected in these children were caused not by an unusual, emerging virus, but by common childhood viral pathogens," said Charles Chiu, Professor of laboratory medicine and medicine in the Division of Infectious Diseases at the varsity. "That's what led us to speculate that the timing of the outbreak was probably related to the really unusual situations we were going through with Covid-19 related school and daycare closures and social restrictions," Chiu said. "It may have been an unintended consequence of what we have experienced during the last two-to-three years of the pandemic." To conduct the study, which was backed by the Centers for Disease Control and Prevention (CDC), researchers used polymerase chain reaction (PCR). In addition, various metagenomic sequencing and molecular-testing methods were also used to examine plasma, whole blood, nasal swab and stool samples from 16 paediatric cases in six US states -- Alabama, California, Florida, Illinois, North Carolina and South Dakota -- from October 1, 2021 to May 22, 2022. The specimens were compared with 113 control samples. In genotyping the 14 available blood samples, AAV2 was detected in 93 per cent of the cases and human adenoviruses (HAdVs) were found in all the cases; a specific type of adenovirus linked to gastroenteritis (HAdV-41) was found in 11 cases. Additional co-infections with Epstein-Barr, herpes and enterovirus were found in 85.7 per cent of cases. Chiu noted the results also mirrored the findings of two concurrent studies conducted in the UK, which identified the same AAV2 strain. All three studies identified co-infections from multiple viruses, and 75 per cent of the children in the US study had three or four viral infections. Since AAVs are not considered pathogenic on their own, a direct causal link with the severe acute hepatitis has yet to be established. The study notes, however, that children may be especially vulnerable to more severe hepatitis triggered by co-infections. While infections from adeno-associated viruses can occur at any age, the peak is typically between 1 and 5 years old, and the median age of the affected children in the study was 3 years old. The clusters of acute severe hepatitis in children have recently waned, but Chiu said the best way to protect children from this unlikely outcome is by washing hands frequently and staying home when sick. /IANS