More than three-quarters of people who have developed severe lung illness after vaping reported using THC-containing products, a new report found, as officials continue to piece together a picture of the mysterious disease.

The new report by the Centers for Disease Control and Prevention said 76.9% of the 514 patients studied used products containing THC, the psychoactive ingredient in marijuana, in the month preceding the onset of symptoms. More than half of the patients reported using nicotine-containing products, while...