A mysterious inflammatory syndrome linked to COVID-19 that's previously been reported in children is affecting adults too, it turns out.
Multisystem inflammatory syndrome in children (MIS-C) is similar to Kawasaki disease, which is rare but can cause irreversible heart damage.
First reported in New York and Italy, it's not confirmed yet whether MIS-C is caused by the same virus as COVID-19 or more cases of it are being seen due to the high number of people being hospitalised in the city's huge outbreak.
The Washington Post now reports the syndrome has been seen in teenage and adult patients in their 20s in New York and San Diego - and like COVID-19, it affects them much worse than children.
"The older ones have had a more severe course," pediatric infectious diseases doctor Jennifer Lighter told the Post, adding that it affects multiple organs - not just the heart and blood vessels, like it does in children.
It's suspected the syndrome wasn't diagnosed in adults at first because Kawasaki disease usually only affects children, and doctors who treat adults have probably never seen it before.
Prof Jane Burns of the Kawasaki Disease Research Center at University of California said it's also more difficult to get a good look at adults' hearts as their chest walls are thicker.
Strangely, most of the adult patients showing signs of MIS-C don't have active infections of COVID-19 - but do have antibodies, suggesting they've either recovered from the disease, or have been exposed to it without getting sick.
Doctors told the Post they suspect it could be the result of the body's immune response, just a few weeks too late. While most COVID-19 infections can be handled at home, everyone who develops MIC-S needs hospital care, they said.
After being noticed in New York, the city hardest-hit by COVID-19 in the world, cases of MIC-S in children have since been reported in Europe. No one's quite sure why some kids develop it while most don't.
"There are no obvious comorbidities as have been observed in adults," Damien Bonnet, a French professor of congenital and pediatric cardiology, told Reuters.
"Genetic susceptibility to 'overreact' to the infection with exaggerated inflammation is probably the mechanism. Research is ongoing to test this hypothesis."