Copyright ©The Author(s) 2021. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Meta-Anal. Aug 28, 2021; 9(4): 353-376
Published online Aug 28, 2021. doi: 10.13105/wjma.v9.i4.353
Systematic review with meta-analysis of the epidemiological evidence in Europe, Israel, America and Australasia on smoking and COVID-19
Peter Nicholas Lee, Janette S Hamling, Katharine Jane Coombs
Peter Nicholas Lee, Katharine Jane Coombs, Department of Statistics, P.N. Lee Statistics and Computing Ltd., Sutton SM2 5DA, United Kingdom
Janette S Hamling, Department of Statistics, RoeLee Statistics Ltd., Sutton SM2 5DA, Surrey, United Kingdom
Author contributions: Lee PN conceived the study, checked the data entry and analysis, and wrote the paper; Coombs KJ carried out the literature searches and assisted in finalizing the paper; Hamling JS checked the literature searches, carried out the data entry and analysis, and assisted in finalizing the paper.
Supported by British-American Tobacco (Investments) Ltd., No. 5700014230; Japan Tobacco International S.A., No. 4700562109; and Philip Morris Products S.A., No 5700131814.
Conflict-of-interest statement: The authors have carried out consultancy work for many tobacco organizations.
PRISMA 2009 Checklist statement: The authors have read the PRISMA 2020 statement, and the manuscript was prepared and revised according to the PRISMA 2020 statement.
Open-Access: This article is an open-access article that was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution NonCommercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See:
Corresponding author: Peter Nicholas Lee, MA, Director, Senior Statistician, Department of Statistics, P.N. Lee Statistics and Computing Ltd., 17 Cedar Road, Sutton SM2 5DA, United Kingdom.
Received: April 3, 2021
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
Research background

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.

Research motivation

We wished to gain a detailed insight into the effect of smoking on a variety of endpoints in different populations.

Research objectives

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.

Research methods

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.

Research results

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.

Research conclusions

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.

Research perspectives

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.