The coronavirus disease (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has caused more than 470 million confirmed cases of COVID-19, and more than 6 million deaths around the world by March 25, 2022 (WHO, 2022). Since the beginning of the COVID-19 pandemic, several factors have been attributed to increasing the risk of infection and adverse outcomes of the COVID-19 disease. Among these factors, the relationship between tobacco use and COVID-19 infection and adverse disease outcomes remained controversial as studies kept reporting mixed findings. Early studies reported what seemed to be a protective effect of tobacco use on COVID-19 infection (Haddad et al.; Jiménez-Ruiz et al., 2020), or hospitalizations due to COVID-19 (Farsalinos et al., 2020; Neira et al., 2021). A more recent ongoing living rapid review, this time including a larger selection of studies with different study designs, found that smokers are at reduced risk of SARS-COVID-19 infection compared to non-smokers (Relative risk 0.67, 95% Credible interval 0.60-0.75) (Simons et al., 2021). These findings opened the way for speculations and hypotheses on the potential mechanisms behind this protective role. However, results from most of these studies may be affected by selection bias as they reported findings from clinical samples or bias due to confounding as the structure of these published data permitted only univariate analysis.

Results from studies that suffer from selection bias or bias due to confounding should be handled with caution as they may undermine years of public health education against tobacco use, a major cause of morbidity and mortality worldwide. Moreover, the role of tobacco use in disease progression such as disease requiring hospitalization, ICU, and death remains unclear as most of the previous studies focused more on the association between tobacco use and the risk of infection, but not the adverse outcomes. These facts call for studies that ensure addressing any knowledge gap on the relation between tobacco and COVID-19 by taking into consideration 1) decreasing the risk for confounding and selection bias; 2) increasing precision through a higher sample size, 3) further investigating the association between tobacco use and adverse disease outcomes. In most Nordic countries, the profile of tobacco use in the underlying populations allows the analysis of several types of tobacco use e.g. cigarette smoking and smokeless tobacco (snus) use, enabling further insights into the potential role of nicotine in the association between tobacco use and COVID-19. The use of smokeless tobacco is highly prevalent (even exceeding the prevalence of smoking among men in Sweden and Norway), which will allow us to disentangle a potential role of nicotine in the association between tobacco use and COVID-19.

The investigators propose to examine the associations between tobacco use, COVID-19 infection, and adverse disease outcomes by using pooled population-based data from three Nordic countries, adjusting for potential confounders. The population-based nature of the samples minimizes selection bias Using a pooled analysis will accrue a large sample size and increase the potential for well-powered sub-groups analyses.