SLC6A20 and COVID-19: The association of rs17713054 SLC6A20-LZTFL1 with severe COVID-19 in non-smoking individuals can be explained; on the one hand, smoking itself is a known risk factor for severe COVID-19 due to its upregulation of ACE-2 expression in the lungs, the host receptor for SARS-CoV-2, making smokers more susceptible to the disease (Reddy et al., 2021).