Background/Objectives: According to the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC), an estimated 3–6% of people suffer from post-COVID condition or syndrome (PCS). A subset meets the diagnostic criteria for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Studies have reported that SARS-CoV-2 proteins or RNA can persist after acute infection in serum or tissues, but their role in PCS is unclear. Methods: Here, SARS-CoV-2 spike protein was analyzed in the serum of 121 PCS patients with predominant fatigue and exertional intolerance, of whom 72 met diagnostic criteria for ME/CFS, 37 post-COVID recovered healthy controls, and 32 pre-pandemic healthy controls. Results: Spike protein was detected in the serum of 11% of recovered controls, 2% of PCS patients, and 14% of ME/CFS patients between 4 and 31 months after SARS-CoV-2 infection, but not in pre-pandemic samples. The occurrence and concentration of spike protein did not correlate with infection or vaccination timepoints. In ME/CFS patients, spike protein presence was not associated with the severity of symptoms or functional disability. In 5 out of 22 patients who under-went immunoglobulin depletion, spike protein levels were reduced or undetectable after treatment, indicating binding to immunoglobulins. Conclusions: In summary, this study identified serum spike protein in a subset of patients but found no association with ME/CFS.

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a complex, multisystemic disorder mostly triggered by viral infections, with core symptoms including post-exertional malaise (PEM), fatigue, pain, and cognitive dysfunction. Its prevalence has increased significantly in the context of the coronavirus disease 2019 (COVID-19) pandemic. Despite its severity and impact on patients' quality of life, ME/CFS remains poorly understood. On May 12 and 13, 2025, the 3^rd International Conference hosted by the Charité Fatigue Center brought together nearly 200 researchers from various disciplines on-site, and around 3,700 participants online to discuss recent advances in ME/CFS research, diagnostics, clinical care, and therapeutic trials. The program featured 33 lectures by international experts on key topics such as post-COVID syndrome (PCS), care structures, and pathophysiological mechanisms including cardiovascular dysregulation, immune dysregulation, autoimmune mechanisms, and metabolic dysfunction. In addition, results from clinical trials addressing disease mechanisms, including those specifically targeting autoantibodies, were presented. While public awareness and funding opportunities have increased in the wake of the pandemic and the emergence of PCS, ME/CFS remains severely underresearched. Sustained and adequately funded research efforts are urgently required to advance understanding, identify diagnostic markers, and develop targeted therapeutic interventions.

According to the World Health Organization (WHO) and the Centers for Diseases Control and Prevention (CDC), currently an estimated 3 – 6 % of people suffer from post-COVID condition or syndrome (PCS). A subset meets diagnostic criteria for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Several studies have reported persistence of SARS-CoV-2 proteins or RNA in serum or tissues of both recovered individuals and PCS patients. In this exploratory study, we investigated whether serum spike protein is associated with PCS and whether it correlates with symptom severity and laboratory biomarkers. We analyzed serum spike protein levels in 121 PCS patients following mild-to-moderate COVID-19, 72 of whom met diagnostic criteria for ME/CFS (post-COVID ME/CFS, pcMECFS). Pre-pandemic seronegative healthy controls (ppHC, n = 32) and post-COVID recovered healthy controls (pcHC, n = 37) after SARS-CoV-2 infection were also included in the study. We found persistent serum SARS-CoV-2 spike protein in a subset of pcHC (11 %), PCS non-ME/CFS patients (2 %), and pcMECFS patients (14 %). There was no significant association with disease severity, symptoms, or laboratory markers. The spike protein concentration was independent of the time since last spike exposure (infection or vaccination). In five spike-positive out of a total of 22 patients who underwent immunoglobulin depletion via immunoadsorption (IA), spike protein was reduced or completely removed after treatment, indicating binding to immunoglobulins. In summary, our study identified serum spike protein in a subset of patients after SARS-CoV-2 infection without evidence for a role in the pathogenesis of PCS.