A mysterious and severe outbreak of hepatitis among young kids that was reported across the U.S. last year has been linked to a childhood respiratory virus.

According to a new study, published in the journal Nature, the respiratory virus – adeno-associated virus 2, or AAV2 – was found in 93% of the cases studied.

According to a World Health Organisation report, more than 1,000 children worldwide – at least 350 of them in the U.S. – were diagnosed with hepatitis between April and July 2022. There were 13 deaths and 22 people required liver transplants. Symptoms like severe liver damage in otherwise healthy children and the growing caseload had left scientists in a fix.

The new research noted that AAV2 wasn't acting solo. This common childhood virus needed "helper" viruses – adenovirus or herpesvirus – to activate and affect the liver cells.

Blood and stool tests, as well as liver biopsies, on the affected children further strengthened the theory. The results indicated the infected individuals had the presence of three or more viruses in their system.

The outbreak started right after the COVID-19 lockdowns were relaxed and schools were reopened. Scientists say children may have been exposed to multiple viruses at the same time.

The results were compared to 113 pediatric patients who developed liver problems due to unknown causes. It showed only 4% of the control group had AAV2.

"Our results suggest that co-infection with AAV2 may cause more severe liver disease than infection by an adenovirus or herpesvirus alone," the authors wrote in the study, reported CNN.

Two other studies conducted in the U.K. also found traces of AAV2 in multiple pediatric hepatitis cases. Since the virus can't copy itself, scientists dismissed its chances of directly causing liver damage.

"If AAV2 directly caused hepatitis, one would expect more cases to have been reported," said Dr. Frank Tacke, a gastroenterologist from Germany who was not involved in the research.

The liver cleans toxins out of the blood and fights infection. Credit: Mayo Clinic