Young people, with mild to no symptoms of Covid-19, are dying of strokes related to the virus, doctors have warned.
The experts say Covid-19 seems to be the cause for sudden strokes in adults in their 30s and 40s, who are otherwise healthy.
Doctors in the US also warned that some of those affected have been reluctant to call 911 because they know hospitals are overwhelmed with coronavirus cases.
The new coronavirus appears to cause the blood to clot in unusual ways, leading to strokes.
"The virus seems to be causing increased clotting in the large arteries, leading to severe stroke," Dr Thomas Oxley, a neurosurgeon at Mount Sinai Health System in New York, told CNN.
He says the cases appear in people all under the age of 50, with either mild symptoms of Covid-19 or, in some cases, no symptoms at all.
"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," he added.
"All tested positive. Two of them delayed calling an ambulance."
The doctors stress the strokes are not common in young people.
"For comparison, our service, over the previous 12 months, has treated on average 0.73 patients every two weeks under the age of 50 years with large vessel stroke," the team wrote in a letter to be published in the New England Journal of Medicine.
"The average person who has a large vessel stroke is severely impaired," Oxley said.
"It means it a bigger clot. It includes one of the largest arteries in the brain."
Doctors are warning people to watch for both Covid-19 and stroke symptoms.