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New York doctors are warning that the coronavirus may cause sudden strokes in adults in their 30s and 40s who are not severely sick.

The doctors at Mount Sinai Health System believe there is growing evidence that COVID-19 can cause the blood to clot in unusual ways, resulting in an uptick in strokes among patients who don’t typically suffer from them, CNN reported.

“The virus seems to be causing increased clotting in the large arteries, leading to severe stroke,” neurosurgeon Dr. Thomas Oxley told CNN.

Oxley and his colleagues said they typically record fewer than two strokes per month in people under the age of 50, but in a two-week period during the pandemic, they treated five. Their findings are to be published in a letter in the New England Journal of Medicine.

“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 said.

“All tested positive. Two of them delayed calling an ambulance,” he went on.

The report could be particularly troubling in New York, where officials have advised against calling 911 unless they come down with severe symptoms, like difficulty breathing, in order to not further overwhelm already inundated EMS.

Some, meanwhile, have been reluctant to visit a hospital for fear they might catch the virus there.

Oxley said his team urged people to watch themselves for symptoms of coronavirus and to call 911 if they believe they’ve suffered a stroke.

He referred to the mnemonic device to remember warning signs for strokes, FAST: F for face drooping; A for arm weakness; S for speech difficulty, and T for time to call 911.