Smoking and COVID-19: what we know so far

The ongoing COVID-19 pandemic has placed a spotlight on infectious diseases and their associations with host factors and underlying conditions. New data on the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) virus are entering the public domain at a rapid rate such that their distillation often lags behind. To minimise weak associations becoming perceived as established paradigms, it is imperative that methodologies and outputs from different studies are appropriately critiqued and compared. In this review, we examine recent data on a potential relationship between smoking and COVID-19. While the causal role of smoking has been firmly demonstrated in regard to lung cancer and chronic obstructive pulmonary disease, such associations have the benefit of decades' worth of multi-centre epidemiological and mechanistic data. From our analysis of the available studies to date, it appears that a relationship is emerging in regard to patients with a smoking history having a higher likelihood of developing more severe symptoms of COVID-19 disease than non-smokers. Data on whether COVID-19 has a greater incidence in smokers than non-smokers is thus far, contradictory and inconclusive. There is therefore a need for some caution to be exercised until further research has been conducted in a wider range of geographical settings with sufficient numbers of patients that have been carefully phenotyped in respect of smoking status and adequate statistical control for confounding factors.