Health officials say they believe the “vast majority” of cases in the U.S. vaping illness outbreak have been tied to vitamin E acetate. 

That's a thickening agent added to illicit THC vaping liquids. 

Officials also said Friday that vaping illnesses can get worse, even deadly, after patients leave the hospital and doctors should check on patients within two days of sending them home. 

The nation's outbreak of vaping-related lung injuries continues. But new cases are on the decline. 

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More than 2,500 cases have been reported across the nation. There have been 54 deaths.

The CDC previously announced that one vape brand was cited by more than half of patients suffering vaping-related illnesses. Dank Vapes was the brand cited by more than half of patients. But it's not a licensed product coming from one business: It's empty packaging that can be ordered from the internet. 

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Previously reported data suggests the vaping outbreak might have peaked in mid-September. The CDC said that since Sept. 15 there has been a steady decline in the number of hospitalized patients reported weekly to the agency. 

Vitamin E is a common ingredient in nutritional supplements and topical skincare treatments. It's usually not harmful when ingested or applied topically, but experts said research indicated the oily nature of the chemical could irritate the lungs when inhaled. The compound is used as a thickener in vaping liquid, particularly in black market vape cartridges. 

Consumers have been urged to avoid e-cigarette products containing THC in particular, especially those obtained through informal means, such as through friends or dealers. 

Vaping illnesses
FILE - In this Friday, Oct. 4, 2019 photo, a man using an electronic cigarette exhales in Mayfield Heights, Ohio. Vaping-related illnesses in the U.S. are still rising, though at a slightly slower pace. On Thursday, Oct. 17, 2019, the Centers for Disease Control and Prevention said there have been nearly 1,500 cases and at least 33 deaths in the still mysterious outbreak. (AP Photo/Tony Dejak)
AP