Published online Aug 28, 2021. doi: 10.13105/wjma.v9.i4.353
Peer-review started: April 3, 2021
First decision: June 18, 2021
Revised: June 28, 2021
Accepted: August 23, 2021
Article in press: August 23, 2021
Published online: August 28, 2021
Previous meta-analyses relating smoking to coronavirus disease 2019 (COVID-19) are limited by considering few studies, restricting attention to hospitalized patients, giving limited or no attention to the definition of smoking or the reliability of smoking as recorded, and failing to properly consider the effect of adjustment for demographics and comorbidities.
We wished to gain a detailed insight into the effect of smoking on a variety of endpoints in different populations.
To carry out a systematic review, based on epidemiological studies in Europe, Israel, America and Australasia on the relationship of smoking to being tested for COVID-19, being positive for COVID-19, being hospitalized with COVID-19, having severe disease or dying.
Literature searches based on publications in English up to September 30, 2020 identified studies of at least 100 individuals, carried out in Europe, Israel, America and Australasia, and unrestricted to those with specific other diseases, and providing information relating smoking to various COVID-related endpoints. Fixed-effect and random-effects meta-analyses were conducted for combinations of index of smoking, endpoint, population and level of adjustment with heterogeneity studied by level of adjustment, study location, and other factors.
Data were available from 74 studies of highly variable size: 37 in the United States, 10 in the United Kingdom, and up to four elsewhere, with populations most commonly studied being those hospitalized with COVID-19, positive for COVID-19, tested for COVID-19 and the general population. Only 36 studies distinguished current and former smokers, and adjusted results for smoking were only given in 42 studies. Positivity for COVID-19 was reduced among smokers in those tested, but not in the general population. Apparent increases in risk in smokers of hospitalization for COVID-19 among those positive, and of death among those positive and among those hospitalized disappeared following adjustment for pre-existing comorbidities, and there was little evidence of any relationship of smoking with admission to intensive care, being mechanically ventilated or having severe COVID-19, even in the unadjusted results.
There is some evidence that smoking is associated with a reduced risk of being COVID-19 positive. Any apparent adverse effects of smoking on hospitalization rates among those positive, and on death rates seem due to the poorer prior health status of smokers.
Evidence from later studies could consolidate these conclusions, and help to explain why, among those tested for COVID-19, current smokers are less likely to be positive.