Smoking and the risk of COVID-19 in a large observational population study

BACKGROUNDSmokers are generally more susceptible to infectious respiratory diseases and are at higher risk of developing severe complications from these infections. Conflicting reports exist regarding the impact of smoking on the risk of Coronavirus disease 2019 (COVID-19). METHODSWe carried out a population-based study among over 3,000,000 adult members of Clalit Health Services, the largest health provider in Israel. Since the beginning of the disease outbreak, and until May 16, 2020, over 145,000 adults underwent RT-PCR testing for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), and 3.3% had positive results. We performed a case-control study among patients who underwent SARS-CoV-2 testing, to assess the impact of smoking on infection incidence and severity. Individuals with positive tests were matched in a 1:5 ratio to individuals tested negative, of the same sex, age, and ethnicity/religion. Conditional logistic regressions were performed to evaluate odds ratios for current and previous smoking on the risk of testing positive. Multivariable logistic regressions were performed among patients infected with COVID-19 to estimate the association between smoking and fatal or severe disease requiring ventilation. Regressions were performed with and without adjustment for preexisting medical conditions. RESULTSCurrent smokers (9.8%) were significantly less prevalent among members tested positive compared to the general population (19.4%, P<0.001), and to matched members tested negative (18.2%, P<0.001). Current smoking was associated with significantly reduced odds ratio (OR) for testing positive OR=0.447 (95% confidence interval (CI) 0.400-0.501). Among patients tested positive, there was no evidence of significantly increased risk of developing severe or fatal disease. CONCLUSIONThe risk of infection by COVID-19 appears to be reduced by half among current smokers. This intriguing finding may reveal unique infection mechanisms present for COVID-19 which may be targeted to combat the disease and reduce its infection rate.