U.S. health officials cast doubt on COVID-19 as a potential cause of severe hepatitis that’s been seen in dozens of previously healthy children around the world, while adding weight to the possibility it’s caused by a more common virus linked to stomach ailments.

The Centers for Disease Control and Prevention on Friday released its most detailed report yet on nine cases of pediatric hepatitis in Alabama that have captured national attention. All the patients tested negative for COVID-19 at the hospital and had no documented history of infection with SARS-CoV-2, the report said.

COVID has shown it can damage a variety of organs, including the liver, raising the possibility that it could be linked to the more than 160 worldwide cases seen so far of unexplained liver disease in children. U.K. experts said earlier this week that the condition might be connected to adenoviruses, a family of pathogens that more commonly cause cold and flu symptoms.

In their description of the Alabama cluster last week, health officials noted that several children had tested positive for adenovirus type 41, which usually causes pediatric acute gastroenteritis — sometimes called stomach flu — leading to nausea, vomiting, diarrhea and sometimes more severe symptoms. Friday’s report confirmed that all five patients whose samples were sequenced showed signs of the same virus type, raising the specter of a possible causal link.

Adenovirus is known to cause hepatitis in immunocompromised children, but in light of the new findings, the CDC report notes, “it might be an underrecognized contributor to liver injury among healthy children.” Adenovirus should be considered among possible explanations for these cases, but the extent of the relationship remains under investigation, the report said.

The disorder has been seen mainly in children younger than 10 and has left a few needing liver transplants. Officials in Wisconsin said earlier this week that one of the cases under investigation resulted in a fatality. So far, no other U.S. cases linked to the disorder have resulted in death.

A separate report from the World Health Organization said it’s unlikely that COVID vaccination could be the cause, as the majority of kids haven’t been immunized. None of the cases have been traced to a family of viruses known to cause acute hepatitis, but more than 75% have tested positive for adenovirus.

Health officials in eight states have confirmed or are investigating cases matching the CDC’s initial description of those in Alabama. Most of the children also tested positive for adenovirus, state health officials say.

Public health officials in California and Colorado said in emailed statements that they are now investigating cases. Delaware, North Carolina and Illinois earlier confirmed cases. Health officials in New York state and Wisconsin also have said they’re investigating reports that match the CDC’s description.

The CDC has not yet responded to requests asking for additional information about how it plans to confirm the cases reported by health departments. However, the additional clinical details included in Friday’s report contain some information health experts have said could help nail down which cases match those found in Alabama.

Hepatitis doesn’t occur frequently in kids, but is not necessarily rare, said Saul Karpen, a pediatric liver expert at Children’s Healthcare of Atlanta and board member of the American Association for the Study of Liver Diseases. Cases caused by known liver viruses are usually reported to local health departments and tracked by the CDC.

Karpen said that about 10% of his pediatric patients with liver transplants have disease that wasn’t caused by one of the recognized liver viruses. What’s remarkable about these liver patients with unexplained disease, he said, is that so many of them have adenovirus 41, and that the cases are so severe.

“The balance between alarm and concern here is real,” Karpen said. While the cases are definitely cause for heightened awareness, he does not think parents should yet be worried.

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