An additional 71 cases of children with hepatitis, or inflammation of the liver, have been reported in the last two weeks, bringing the total to 180 patients under investigation in 36 states, the CDC said in a Wednesday news release.

While the increase is considerable, many reported cases are “retrospective,” meaning that they occurred sometime since October but not necessarily recently, according to the health agency.

“Most of the numbers involve patients that are just now being reported, rather than new cases of hepatitis … and some may ultimately wind up not being linked to this current investigation,” the press release stated.

There have been no U.S. deaths reported in conjunction with hepatitis cases since February, and the proportion of U.S. patients needing liver transplants dropped 6% to 9% over the past two weeks, the agency added.

As of April 23, at least 160 cases had been reported in Europe, with approximately 10% requiring liver transplants and at least one death reported, according to the WHO.

A COVID link?

The cause of the mysterious cases of liver inflammation, some requiring transplant surgery, is currently unknown. Medical officials have been quick to implicate adenovirus, a common virus that causes cold- or flu-like symptoms, as a potential cause, as it’s been detected in nearly half of children with hepatitis of unknown cause. Other potential culprits include COVID, and hepatitis A, B, C, D, and E viruses, according to the CDC.

But adenovirus “does not fully explain the severity of the clinical picture,” according to a late April report by the World Health Organization: “Infection with adenovirus type 41, the implicated adenovirus type, has not previously been linked to such a clinical presentation.”

Usual symptoms of adenovirus type 41 include diarrhea, vomiting, fever, and respiratory symptoms, according to the WHO, and the strain isn’t known to cause hepatitis in otherwise healthy children.

Additional potential causes of the mysterious hepatitis, such as a new adenovirus or COVID-19 co-infection, must be further investigated, the report added.

There is abundant evidence that prior COVID infections can lead to damage in multiple organ systems, Arijit Chakravarty, a COVID researcher and CEO of Fractal Therapeutics, told Fortune on Wednesday, ahead of the CDC’s update. 

Chakravarty is among the COVID experts who think the coronavirus is worth a more serious look as a potential cause or cofactor. A November 2020 study published in Hepatology concluded that hepatitis is common in children with MIS-C, a rare, inflammatory-based illness that can occur in children after COVID infection or exposure.

Researchers should look not just for active COVID infections in children with hepatitis of unknown origin, but for blood markers of prior infection, Chakravarty said.

“Is it possible it’s not COVID? Of course it’s possible,” he said. “But you need to rule it out specifically first. And you don’t need to differentiate a trigger from a direct cause in order to take a practical course of action from it.”

Dr. Alexandra Brugler Yonts, an infectious disease specialist and director of the Pediatric Post-COVID Program at Children’s National in Washington, D.C., agreed that a potential COVID link “needs to be explored.”

“But it should not be the only focus,” she told Fortune, “especially since these cases of hepatitis emerged after the world opened up again and all the other viruses came back to the party.”