Yesterday, the World Health Organization said it had received reports of 650 possible cases of acute hepatitis in children, but added that its causes were not yet known and were still under investigation.

An organization’s study showed that as of May 26, 33 countries had notified it of a total of 650 possible cases, with an additional 99 cases classified as pending.

Health authorities around the world are looking into a mysterious increase in cases of chronic hepatitis in children, which has led to at least nine deaths.

U.S. health officials said last week that adenovirus infection, a common childhood virus, is the leading hypothesis for recent cases of chronic hepatitis of unknown origin in children.

Hepatitis associated with this type of adenovirus almost only affects immunocompromised children.

The U.S. Centers for Disease Control and Prevention also said they are looking at whether Covid-19 infection plays a role in this framework, in addition to other pathogens, drugs and risk factors.