MMP9 and COVID-19: Regardless of the unchanged values of BDNF in different phases of COVID-19, we found a positive correlation between BDNF and MMP-9, MMP-8, and VEGF A, as well as a strong negative correlation of BDNF with IL-10 and cytokines ratios IL-10/IL-1, IL-10/IL-6, IL-10/IL-17, and IL-10/TNF-α (Table 3).