At least 285 U.S. children have developed a serious inflammatory condition linked to the coronavirus and while most recovered, the potential for long-term or permanent damage is unknown, two new studies suggest.

The papers, published online Monday in the New England Journal of Medicine, provide the fullest report yet on the condition.

The condition is known as multisystem inflammatory syndrome in children. It is considered uncommon and deaths are rare; six children died among the 285 in the new studies.

Including cases in Europe, where it was first reported, about 1,000 children worldwide have...