COVID-19 and smoking: A systematic review of the evidence

Abstract

COVID-19 is a coronavirus outbreak that initially appeared in Wuhan, Hubei Province, China, in December 2019, but it has already evolved into a pandemic spreading rapidly worldwide 1,2 .As of 18 March 2020, a total number of 194909 cases of COVID-19 have been reported, including 7876 deaths, the majority of which have been reported in China (3242) and Italy (2505) 3 .However, as the pandemic is still unfortunately under progression, there are limited data with regard to the clinical characteristics of the patients as well as to their prognostic factors 4 .Smoking, to date, has been assumed to be possibly associated with adverse disease prognosis, as extensive evidence has highlighted the negative impact of tobacco use on lung health and its causal association with a plethora of respiratory diseases 5 .Smoking is also detrimental to the immune system and its responsiveness to infections, making smokers more vulnerable to infectious diseases 6 .Previous studies have shown that smokers are twice more likely than non-smokers to contract influenza and have more severe symptoms, while smokers were also noted to have higher mortality in the previous MERS-CoV outbreak 7,8 .Given the gap in the evidence, we conducted a systematic review of studies on COVID-19 that included information on patients' smoking status to evaluate the association between smoking and COVID-19 outcomes including the severity of the disease, the need for mechanical ventilation, the need for intensive care unit (ICU) hospitalization and death.The literature search was conducted on 17 March 2020, using two databases (PubMed, ScienceDirect), with the search terms: ['smoking' OR 'tobacco' OR 'risk factors' OR 'smoker*'] AND ['COVID-19' OR 'COVID 19' OR 'novel coronavirus' OR 'sars cov-2' OR 'sars cov 2'] and included studies published in 2019 and 2020.Further inclusion criteria were that the studies were in English and referred to humans.We also searched the reference lists of the studies included.A total of 71 studies were retrieved through the search, of which 66 were excluded after full-text screening, leaving five studies that were included.All of the studies were conducted in China, four in Wuhan and one across provinces in mainland China.The populations in all studies were patients with COVID-19, and the sample size ranged from 41 to 1099 patients.With regard to the study design, retrospective and prospective methods were used, and the timeframe of all five studies covered the first two months of the COVID-19 pandemic (December 2019, January 2020).Specifically, Zhou et al. 9 studied the epidemiological characteristics of 191 individuals infected with COVID-19, without, however, reporting in more detail the mortality risk factors and the clinical outcomes of the disease.Among the 191 patients, there were 54 deaths, while 137 survived.Among those that died, 9% were current smokers compared to 4% among those that survived, with no statistically significant difference between the smoking rates of survivors and