The cases involve previously healthy children up to the age of 16, many of whom are suddenly developing hepatitis and displaying gastrointestinal symptoms including abdominal pain, diarrhea, and vomiting, as well as jaundice, or yellowing of the skin and eyes.

Two of those children, one of whom is an infant and the other who is 2, have required liver transplants, he said. One of them is still waiting.

Researchers believe the outbreak of severe acute hepatitis may be linked to adenovirus, a viral infection that usually causes the common cold.

“We do not know yet if adenovirus played a role in these rare illnesses or if these cases are connected,” Ali Bay, a spokeswoman for the California Department of Public Health, told the Chronicle.

“While only a few cases of this rare illness have been reported, we urge parents and guardians to take common-sense general measures to prevent infection and illness, such as good hand hygiene, covering coughs and sneezes, and keeping children home from school or childcare while they are ill.”

In its health alert last month, the CDC urged physicians to be on the lookout for symptoms and to report any suspected cases of unknown origin to their local and state health departments.

The health agency said it’s also working with state health departments to see if there are additional U.S. cases, and what may be the cause.

Meanwhile, the World Health Organization has reported at least 169 cases in 12 countries as of April 21, including in the United Kingdom, Spain, Israel, Denmark, Ireland, the Netherlands, Italy, Norway, France, Romania, Belgium, and the United States.

Adenovirus has been detected in at least 74 cases, officials added.

“While adenovirus is a possible hypothesis, investigations are ongoing for the causative agent,” the health body stated, noting that this hypothesis “does not fully explain the severity of the clinical picture” and that “the implicated adenovirus type, has not previously been linked to such a clinical presentation.”

The agency noted that certain factors such as increased susceptibility among young children following a lower level of circulation of adenovirus during the COVID-19 pandemic, the potential emergence of a novel adenovirus, and COVID-19 infection still need to be investigated.

However, it ruled out any potential links to COVID-19 vaccines, saying that the vast majority of the affected children with acute hepatitis weren’t vaccinated.