Blood thinners don’t reliably prevent clotting in people with COVID-19, and young people are dying of strokes caused by the blockages in the brain. And many people in hospital have drastically elevated levels of a protein fragment called D-dimer, which is generated when a clot dissolves. High levels of D-dimer appear to be a powerful predictor of mortality in hospitalized patients infected with coronavirus3.

Researchers have also observed miniature clots in the body’s smallest vessels. Jeffrey Laurence, a haematologist at Weill Cornell Medicine in New York City, and his colleagues examined lung and skin samples from three people infected with COVID-19 and found that the capillaries were clogged with clots4. Other groups, including a team led by O’Donnell, have reported similar findings5.

“This is not what you’d expect to see in someone who just has a severe infection,” he says. “This is really very new.” This might help to explain why some people have critically low blood-oxygen readings, and why mechanical ventilation often doesn’t help. It’s a “double hit”, says O’Donnell. Pneumonia clogs the tiny sacs in the lungs with fluid or pus, and microclots restrict oxygenated blood from moving through them.