ACE2 and infection: Published research29 indicated that smokers are twice as likely to develop severe COVID-19 as nonsmokers, as smoking tobacco induces alterations in the respiratory tract and cell-mediated immune response.29 Furthermore, current smokers have higher ACE-2 gene expression than non-smokers; this gene is responsible for ACE-2 receptors production, which COVID-19 attaches to and penetrates the cells, thus raising the chance of infection.30 Our study did not find an association between smoking habits and the duration of ICU stay or clinical prognosis.