DPP4 and COVID-19: Applying advanced molecular profiling in searching for the singular biomarker of post-COVID-19 fatigue and neurovascular dysfunction, specifically those related to DPPIV and other proteolytic enzyme pathways by using techniques such as single-cell RNA sequencing, metabolomics, proteomics, whole-genome sequencing, and gut microbiome studies offer promising avenues to unravel the pathophysiological processes underlying post-COVID-19 fatigue.