The outbreak of the novel coronavirus (SARS-CoV-2)-infected disease (COVID-19) began inDecember 2019, spread throughout China in early 2020 and developed as a pandemicthereafter.Based on current knowledge, Covid-19 infection causes mild to moderate respiratory andgastrointestinal symptoms in the majority of patients. In a smaller percentage severedisease courses are observed, often with the need of hospitalization and intensive caretreatment. Apparently, symptoms can persist for relatively long time after viralclearance, suggesting the existence of a "Post-Covid" syndrome. A study from the UKidentified fatigue, breathlessness and psychosocial stress as common symptoms afterdischarge from the hospital. Covid-19 infection is frequently characterized by ahyperinflammatory phenotype and a cytokine storm. The Covid-19 cytokine storm ischaracterised by rapid proliferation and hyperactivation of T cells, macrophages, mastcells, neutrophil granulocytes and natural killer cells, and the overproduction ofinflammatory cytokines and chemical mediators released by immune or nonimmune cells.Early data also suggest that even if symptoms are just 'mild to moderate' during theacute infection, fibrotic lung damage develops in some patients. This may lead tolong-term pulmonary complications for a subset of patients. The mechanisms for post-Covidpulmonary fibrosis are still unclear: inflammation triggering fibrosis, epithelial andendothelial injury with inadequate fibroproliferation and vascular damage are consideredto be possible mechanisms.A potential therapeutic target in ameliorating post-Covid symptoms could be the gutmicrobiome. Gut microbiome alterations have been described in Covid-19. The gut-lung axisas a link between dysbiosis, barrier dysfunction, translocation of bacterial products andhyperinflammation has been proposed as a potential therapeutic target. Probiotics havebeen proposed to be a possible modulator of the deranged gut-lung axis in Covid-diseaseand post-Covid syndrome. Currently 11 studies are registered in clinicaltrials.gov fortreatment of acute Covid disease and prevention of the disease (including one study fromGraz), but no study related to post-Covid syndrome could be found.Therefore, it is currently unclear, which clinical, immune system or microbiome relatedbiomarker would be the best to study the effect of a microbiome-based intervention inpost-Covid syndrome.