Numerous lines of evidence suggest that the influence of various factors on susceptibility, hospitalization, severity, and death from COVID-19 is a very complex and multifaceted issue that requires a comprehensive understanding of the interplay between the following items: 1) individual factors such as age, occupation, workplace, pre-existing diseases and lifestyle; 2) social factors such as socioeconomic status and the Human Development Index (HDI) of the country of residence, access to medical and nursing services, hospital facilities, etc.; 3) host genetic polymorphisms; and 4) genetic mutations in the SARS-CoV-2 virus. The present study focused on host’s genetic polymorphisms.
The present study identified a total of 177, 91, 67, 67, and 22 polymorphic protein-coding loci associated with susceptibility to, hospitalization for, severity of, and death from COVID-19, respectively. It was hypothesized that the mentioned genes were enriched in some biological pathways. Unexpectedly, no significant results were found in the enrichment analysis. It seems that the present results did not support our hypothesis. On the other hand, the low number of common genes among the four sets of susceptibility, hospitalization, disease severity, and death due to COVID-19 indicates the very high diversity in the disease course and the involvement of factors other than the genetic background of the host, including many environmental factors, in the disease and its outcome. It appears that susceptibility, hospitalization, disease severity, and death due to COVID-19 are distinct multifactorial traits.
As mentioned in the Results section, susceptibility to, hospitalization for, severity of, and death from COVID-19 shared 39, 22, 19, and 9 loci, respectively, with the smoking initiation trait. Statistical analysis indicated that the levels of sharing were highly significant (see Table 3). Several risk factors associated with susceptibility, hospitalization, severity, and death from COVID-19, including chronic medical conditions (such as hypertension, heart failure, coronary artery disease, diabetes mellitus, and cancer), smoking habit, and obesity (Docherty et al., 2020), have been identified from the epidemiologic evidence accumulated during the COVID-19 pandemic. Almost from the beginning of the pandemic, studies have emphasized the role of smoking habits and their relationship with susceptibility to infection with SARS-CoV-2 virus, hospitalization, severity of illness, and death due to COVID-19. Interestingly, epidemiological studies have reported that smoking habit is an important risk factor for many respiratory infections, including influenza, Middle East Respiratory Syndrome (MERS), and COVID-19, and may also accelerate disease progression in patients [2, 33-36].
Smoking is a major risk factor for many diseases. Because of its high prevalence in human populations and its positive association with a wide range of diseases, it remains a serious health concern. Smoking initiation has been shown to have significant heritability [37], and many polymorphic loci have been identified that show significant association with the trait [38]. It should be noted, however, that this is a modifiable trait.
Evidence suggests that smoking initiation is significantly associated with increased risk of cancer (e.g., breast, lung, colorectal, etc.) ([3-41], 14 different cardiovascular diseases (e.g., coronary heart disease, heart failure, ischemic stroke, transient ischemic attack, and arterial hypertension [42], atrial fibrillation[43], type 2 diabetes mellitus [44], increased abdominal fat distribution [45], and so on.
Finally, I would like to draw your attention to the following evidence.
1. Smoking initiation is associated with susceptibility to, hospitalization for, severity of, and death from COVID-19.
2. Shared genes between smoking initiation and susceptibility to, hospitalization for, severity of, and death from COVID-19.
3. Associations between smoking and chronic diseases related to COVID-19, such as cancer, diabetes mellitus, cardiovascular disease, etc.
Taken together, it is reasonable to conclude that factors such as chronic cardiovascular disease and diabetes mellitus are not independent risk factors for COVID-19 and its outcomes, and that smoking is the most important and influential risk factor. To confirm or reject this conclusion, further analysis using Mendelian randomization on the available information is needed.