Mitochondrial Function in Peripheral Blood Mononuclear Cells, Muscle, and Skin of Long COVID Patients

Mitochondria are structures inside cells that are responsible for producing energy from nutrients through a series of steps, using oxygen. To have an idea of how well the mitochondria can produce energy, we can measure how much oxygen they use. The goal of this study is to: 1. To compare the oxygen use between people with long COVID and people who completely recovered after COVID-19, using three techniques that measure the oxygen use in blood cells, on the skin and in the muscle 2. To test how similar these three techniques are in measuring the oxygen used in the three different tissues Participants will: * Complete surveys, * Wear an activity tracker (7 days), * Undergo several non-invasive tests, and * Donate a blood sample The study will take place in UZ Brussel hospital and will take 2 study visits, approximately 7 days apart.