A hidden physical change in the body may be helping to drive the prolonged malaise some people experience after contracting COVID-19.

Analyzing blood samples from patients with long COVID, a team of medical researchers has identified unusual microscopic structures that may contribute to symptoms such as brain fog and fatigue. If this is the case, it offers a hopeful target for future treatment.

“This study shows a robust association between biomarkers indicative of thromboinflammatory activity and long COVID,” the team writes in a paper led by geneticist Alain Thierry of Montpellier University in France.