The novel coronavirus is causing strokes in some younger adults who are otherwise asymptomatic or have mild symptoms, doctors who have treated the virus said Wednesday.

Thomas Oxley, a neurosurgeon at New York’s Mount Sinai Health System, told CNN about five partially or wholly asymptomatic coronavirus patients under the age of 50 whom he and his colleagues treated. He said the patients seemed to have suffered increased clotting in large arteries, causing severe stroke.

“Our report shows a seven-fold increase in incidence of sudden stroke in young patients during the past two weeks. Most of these patients have no past medical history and were at home with either mild symptoms (or in two cases, no symptoms) of Covid,” Oxley told CNN.

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Strokes in general and large-vessel strokes in particular are not common within the 30-40 age range.

“For comparison, our service, over the previous 12 months, has treated on average 0.73 patients every 2 weeks under the age of 50 years with large vessel stroke,” Oxley and his team wrote in a letter to the New England Journal of Medicine, the outlet noted.

He also said at least two of the patients delayed calling an ambulance, suggesting they did not make the connection to the virus and that they suspected hospitals would be too overwhelmed to treat them. Oxley said delays in such cases could prove fatal.

“The most effective treatment for large vessel stroke is clot retrieval, but this must be performed within 6 hours, and sometimes within 24 hours,” Oxley wrote.

At least one of the patients in question has died, while another has been sent home but still requires intense care, Oxley said. Others have been transferred to intensive care, stroke units or rehabilitation facilities.

Oxley warned people who suspect they may have the virus to be aware of not just coronavirus symptoms but also signs of stroke such as facial drooping, arm weakness and difficulty with speech.