ST. PAUL, Minn. — Minnesota health officials warned Friday of a possible resurgence of lung injuries related to vaping, saying they've had 11 reported cases in the past two months.

The state Health Department asked medical providers to watch for additional cases. Officials also said the coronavirus has made it more difficult to identify vaping-related cases because the symptoms can be similar. All 11 cases — with a median age of 18 — required hospitalization and some needed ventilators.

"This public health threat got a lot of attention last year, and unfortunately it has not gone away," Health Commissioner Jan Malcolm said in a statement.

A national outbreak of lung illnesses last year was largely attributed to people vaping liquids that contain THC, the high-inducing part of marijuana. The Centers for Disease Control eventually said vitamin E acetate, a thickening agent sometimes added to illicit THC vaping liquids, was believed to have caused most of the illnesses.

The Minnesota Health Department said the 11 people suffering lung injuries in recent months sought treatment for cough and shortness of breath, but tested negative for the coronavirus. All said they had a history of vaping, and most reported vaping liquids with THC.

Minnesota had 149 confirmed or probable cases of vaping-associated lung injuries, and three deaths in 2019 before the outbreak tapered off in the fall.

In neighboring Wisconsin, state health officials said Friday they had received three reports of vaping-associated lung injuries since May. In 2019, the state received reports of 103 such injuries, while there have been just 10 reports in all of 2020.