It is not clear whether any of the patients who relapsed or died had started vaping again when they got home, Dr. Schuchat said, but she said that it was important for addiction counseling to start before patients leave the hospital.

She added: “I think it’s likely that there are a variety of factors here, and the medical conditions were particularly important.”

Another study addressed the lingering question of whether the outbreak, first widely recognized in August, was really a new illness, or actually something that had been going on for a long time without being detected.

The illness appears to be new: Cases spiked in early June, the researchers found, based on analyzing emergency-room visits reported to a database called the National Syndromic Surveillance Program, which was created to detect bioterrorism after the attacks on Sept. 11, 2001. They examined the reasons for visits, looking for mention of e-cigarettes, along with lung symptoms like shortness of breath and chest pain.

Why the surge occurred in June is not known, though researchers suspect it was because of increased use of vitamin E acetate, and other potentially toxic additives, by suppliers of illicit THC vaping products. Minnesota authorities who seized illicit THC vapes found that in 2018, 10 of 10 products contained no vitamin E, whereas 20 of 20 seized in September 2019 did contain it.

“I believe the practice of diluting THC vaping products with vitamin E really took off this past year,” Dr. Schuchat said, adding that the idea of using the vitamin additive had been promoted on YouTube and other social media platforms.

But how to explain the gradual increase in emergency room visits by young patients 10 to 19 years old starting in January 2017, well ahead of the outbreak? Though some cases could be linked to whatever later caused the severe illnesses, the researchers said the problems could also have been the result of the increasing use of nicotine e-cigarettes. Among high school students, the proportion who said they had vaped nicotine in the previous 30 days rose to 27.5 percent in 2019, from 11.7 percent in 2017.