A study published in the scientific journal Nature may provide some clues as to what is causing a mysterious outbreak of a severe liver disease in otherwise healthy children.

Health experts have been stumped after over 1,000 children around the globe were diagnosed with cases of hepatitis starting in 2021. In the United States, there have been nearly 400 unexplained cases of hepatitis among children. According to the Centers for Disease Control and Prevention, 22 children required liver transplants, and 13 died.

Now, scientists are one step closer to finding the cause of the outbreak. Researchers found that a vast majority of children were infected with adeno-associated virus 2 (AAV2), which is not known to cause illness.

The study found that in addition to AAV2, many of the children were also infected with three or four common viruses.

“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 Dr. Charles Chiu, senior author of the paper.

Researchers believe that the outbreak could be the result of closing down schools during the coronavirus pandemic. With schools shut down and students learning remotely, they were not exposed to many common viruses, preventing their immune systems from building defenses against them. When they returned to class, they were suddenly exposed to a host of new viruses.

“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.”

For now, scientists say that more research is needed to find the exact cause of the hepatitis outbreak.