sElective Serotonin reuPtake inhibitoRs In posT-covid After COVID-19

In this randomized placebo-controlled trial, the investigators will study the effectiveness of fluvoxamine in reducing fatigue severity (primary outcome), cognitive problems, PEM and POTS after 12 weeks of treatment in 160 post-COVID patients. Moreover, the investigators will study treatment-emergent changes in plasma biomarkers, including blood-based neuro)inflammatory markers, cortisol, serotonin, aryl hydrocarbon receptor -indoleamine 2,3-dioxygenase-2 (IDO-2) and kynurenine pathway (KP) metabolites for potential mechanistic pathways of post-COVID treatment. Numerous studies have indicated involvement of brain dysfunction in post COVID, which also relate to the degree of symptom severity (e.g. fatigue / cognitive problems). In an optional neuro-imaging sub-study, the investigators will use functional neuroimaging techniques with and without cognitive challenge to gain a better understanding of the brain functioning and structure in long COVID during fluvoxamine treatment versus placebo. Objectives: * To determine if fluvoxamine treatment (50 mg to 200 mg daily dosing) results in lower levels of fatigue severity than placebo after 12 weeks of treatment (primary). * To determine if fluvoxamine treatment results in lower levels of PEM and POTS and a better cognitive functioning and health-related quality of life (HRQL) than placebo. * To determine if changes in symptoms, i.e. fatigue severity, PEM, POTS, cognitive symptoms, are related to changes in biomarkers, i.e., (neuro)inflammation markers, cortisol, serotonin and IDO-2 -KP metabolites. * To determine if biomarkers, i.e., (neuro)inflammation markers, cortisol, serotonin and IDO-2- KP metabolites, change from baseline to week 12 in participants who received fluvoxamine. Optional Neuro-imaging sub-study: -To determine which changes occur on functional brain imaging, brain metabolites and neuroinflammation during cognitive challenge and to determine if this brain response to cognitive challenge changes after fluvoxamine treatment versus placebo.