(CNN) Health officials in the UK have released new details in their ongoing investigation of an unusual series of hepatitis cases in children. The new report helps explain why they have zeroed in on a possible link to the adenovirus family, the UK Health Security Agency announced Monday.
Since the beginning of the year, at least 111 children have been identified in the UK with acute liver inflammation that does not appear to be caused by the group of hepatitis viruses that would've been a more likely culprit. Many more cases have been announced in the US and other countries around the world.
Roughly three-quarters of the 53 children who were tested for adenovirus in the UK came back positive. The virus that causes Covid-19, on the other hand, was found in only a sixth of children who were tested — in line with the levels of community transmission in the UK.
Adenoviruses make up a large family of viruses that can spread from person to person, causing a range of illnesses including colds, pinkeye and gastroenteritis. They are only rarely reported as a cause of severe hepatitis in healthy people.
But these hepatitis cases come as the spread of adenovirus has escalated in recent months, along with other common viruses that have surged with the end of Covid-19 prevention measures and behaviors that kept most germs at bay.
After falling dramatically during the pandemic, documented adenovirus cases have roared back and are now at higher levels than the UK saw before Covid-19.
Although investigations are circling around adenovirus, how it might cause liver inflammation is still unclear. Experts say the virus may be just one factor that leads to these cases when it happens alongside something else.
"There may be a cofactor causing a normal adenovirus to produce a more severe clinical presentation in young children," the UK health agency said in its technical briefing Monday, "such as increased susceptibility due to reduced exposure during the pandemic, prior SARS-CoV-2 or other infection, or a yet undiscovered coinfection or toxin. Alternatively, there may have been emergence of a novel adenovirus strain with altered characteristics."
Experts say another possibility may be timing. It could be that children who would normally have been infected with minimal symptoms as babies are having more severe reactions to the viruses now that they are older.
UK scientists have their sights on a specific type of adenovirus due to blood sample data, but they will need to look at its genetic makeup as confirmation.
According to the UK Health Security Agency, the cases are largely in kids under 5, with a median age of 3, and only "a small number of children over the age of ten are being investigated." Dozens have recovered and no deaths have been reported in the UK, but 10 children have needed liver transplants.
On Saturday, the World Health Organization said that at least 169 cases of acute hepatitis in children have been identified in 11 countries, including at least 17 liver transplants and one death.
In the United States, potential cases have been identified in Alabama, North Carolina and Illinois.