The Centers for Disease Control and Prevention is investigating 109 cases of severe hepatitis, including five deaths, the public health agency said on Friday.

Ninety percent of the children were hospitalized and 14% required liver transplants, according to the CDC. More than half of the kids had evidence of adenovirus infection. However, public health officials said they don’t know yet if adenovirus is the actual cause.

The cases under investigations were found across 25 states and territories and occurred over the past seven months. CDC officials said they have not documented a significant increase in hepatitis cases or liver transplants, but that’s based on preliminary data and could change. However, the United Kingdom — which first alerted the world to the issue — has documented a significant increase, officials said.

Covid-19 vaccination is not the cause of the illnesses, the CDC officials said.

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