Table 1.

Early and later reviews on tobacco use and COVID-19.

Author	Date	Studies	Conclusions	Notable limitations
Vardavas et al.	March 2020	5 from China	Smoking likely had a negative impact on COVID-19 disease progression and outcomes	Very small sample, very early data
Berlin et al.	April 2020	6 from China	More evidence is needed if tobacco use may be a risk factor for both transmission and negative outcomes	Small sample, early data
Farsalinos et al.	May 2020	13 from China	Smokers are underrepresented among hospitalized patients; former smokers have higher odds of adverse outcomes than current smokers; nicotine may be protective	Relies on early data exclusively from China; unadjusted for confounding factors including sociodemographic factors
Gulsen et al.	June 2020	14 from China, 2 from Italy	History of smoking is associated with severe COVID-19	Relies heavily on early data from China; classifications of both smokers and COVID-19 patients varied
Patanavanich et al.	Sept 2020, preprint	47 from 16 countries	Smoking is an independent risk factor for COVID-19 severity and death	Does not include research published after May 25, 2020; includes studies with very small (>0) smoking populations; no requirement regarding COVID-19 diagnosis
Salah et al.	Oct 2020	10 from various countries (China, UK, Thailand)	Smoking doubles the risk of mortality in COVID-19 patients	Used only PubMed; included mortality as an outcome
Gupta et al.	Nov 2020	23 from various countries (China, US, Italy)	Tobacco use is associated with comorbidities which increase the likelihood of negative COVID-19 outcomes	Literature review; not systematic
Umnuaypornlert et al.	Feb 2021	40 from 8 countries	Tobacco use increases the risk of disease severity and death in COVID-19 patients	No requirement regarding COVID-19 as diagnosis; no minimum number of smokers to be included in studies