The authors identified 19 peer-reviewed papers (16 from China, 1 from Korea, and 2 from U.S.) published before April 28, 2020 on COVID-19 patients with data on smoking behavior. The analysis consisted of 11,590 COVID-19 patients, 731 (6.3%) with a history of smoking. Among the patients with a history of smoking, 29.8% experienced COVID-19 progression, compared to 17.6% non-smoking patients who experienced disease progression (OR 1.91, 95% CI 1.42-2.59). 
*"There was moderate heterogeneity among the studies (I2=38%, p=.048) and no significant evidence of publication bias (Harbord’s p=.813, Peters’ p=.941)."

Smoking Is Associated With COVID-19 Progression: A Meta-analysis

Michigan State University

Several studies (immunohistochemistry and transcriptomic analysis) show that the following relationship exists between ACE2 expression and smokers:

smokers with chronic obstruction pulmonary disease (COPD) > healthy smokers > nonsmokers

Increased expression was noticed in alveolar type 2 progenitor cells (AT 2), alveolar macrophages, and the small airway epithelium.  This phenomenon is thought to play a role in the metabolic changes that lead to the replication of SARS-CoV-2.  Additionally, cigarette smoke is a common risk factor for many other respiratory diseases.  

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