Two U.S. research groups have reported finding nearly 300 cases of an alarming apparent side effect of Covid-19 in children, a condition called multisystem inflammation syndrome, or MIS-C. While researchers have previously reported on the condition, the papers mark the first attempt to measure how frequently the side effect occurs and how it affects children who develop it. In one of the studies, led by researchers at Boston Children’s Hospital, 80% of the children who developed the condition required intensive care, 20% required mechanical ventilation, and four children, or 2%, died. In the second study, from researchers from New York state, a similar percentage of 99 children who developed the syndrome required ICU care and two children died. In both studies, many of the children developed cardiovascular and clotting problems and many had gastrointestinal symptoms. A high proportion also had skin rashes.