BDNF and COVID-19: In summary, we found that BDNF (OR [95% CI] = 0.98 [0.96, 1.00], p = 3.53 ×10-2, proportion = 45.6%) and QPCT (OR [95% CI] = 0.99 [0.99, 1.00], p = 2.09 ×10-2, proportion = 16.8%) partially mediated the causal effect between LTL and COVID-19 susceptibility.