Despite the interest in COVID-19, which is clearly immense when viewing PUBMED for 2020 as shown in Table 1, precise prevalence data on COVID-19 are not only hard to locate, but appear to be constantly changing. In this regard, there appears to be more papers discussing exponential increases in COVID-19 prevalence, rather than decreases over the past year. Selected areas across the globe on the other hand, show few or no cases in rare circumstances as of mid November 30 2020. Others report better success at limiting widespread infections than others. Most cases to date have however, been elderly adults, with or without pre-existing health conditions, regardless of country examined. At the same time, possible additional determinants of COVID-19 risk or susceptibility examined at this time include air pollution, a variety of nutritional factors, and low socioeconomic status. Most data emanate from already infected adults, and thus what is known may be limited to an already impaired elderly subgroup. Data show however, there may be a total of 60 predictors for disease severity, of which seven appear to be highly consistent, while 40 are said to be of medium relevance, and 13 of low relevance. Among those factors that tend to occur with associated consistency other than age are, C-reactive protein levels (a marker of inflammation), D-dimer presence – a prognostic marker released when a blood clot breaks down, albumin, body temperature, a SOFA (sepsis related) score and diabetes 2.

These data stress the need to focus on the elderly, especially those with possible underlying inflammatory, chronic, cardiac and respiratory conditions that may precipitate COVID-19 risk and lethality, and may in turn heighten pre-existing disease symptoms that may be associated in their own right with remediable determining factors 345. These include vitamin D and its impact on innate immunity, as well as the impact of poor air quality, negative health behaviors, and various degrees of mental health status 678910.

While the skeletal and extra skeletal effects of vitamin D have been described and studied for some time, more recent work shows vitamin D to have widespread effects on gene expression, as well as overall health. In the context of respiratory infections such as COVID-19, additional research shows vitamin D can serve to provide a key barrier function within the lining of the respiratory tract. It can also play an essential role in immune system functioning 11. Alternately, a vitamin D deficiency has the potential to increase mortality rates in the elderly with cardiovascular diseases, and various forms of cancer, who often exhibit high rates of vitamin D deficiency, and who are highly susceptible to COVID-19 infections. Vitamin D also has important innate antimicrobial properties 12 that may explain its proposed role in the context of COVID-19 disease 1314.

At the same time, vitamin D production in the skin, which is known to decline with advancing age, possibly renders older populations more dependent on dietary vitamin D than younger populations, in the event of poor air quality or lack of sunlight exposure or both, among other factors. However, this may go undetected, and hence untreated, and thus it is no surprise that vitamin D deficiency is common among older adults in all parts of the world 15, even in sunny climates, where it may predict an older adults’ increased risk of incurring a COVID-19 infection 16.

A vitamin D deficiency has also been found to occur more frequently in adults with diabetes, a leading predictor of COVID-19 severity 1718.

On the other hand, since vitamin D presence is found to decrease the lethality of acute respiratory infections similar to those occurring in persons with COVID-19 infections.COVID-19, it has been suggested that COVID-19 infections may be impacted favorably by efforts to rectify any prevailing hypovitamin D levels among vulnerable adults 192021, especially adults with coexisting diabetes 22. Bielkoski et al. 23 further supports this view given the past evidence for a role of vitamin D deficiency in respiratory tract infections that are more frequent in the winter months and especially in the northern latitudes than they are in summer. Moreover, a low vitamin D status, measured as the plasma level of the transport form of vitamin D, 25(OH)D is widespread worldwide. Table 2.

Pollution, an under-reported possible mediator or moderator of COVID-19 including air pollution, is associated with various forms of allergy, as well as influencing the extent of autoimmunity 33. In addition, empirical evidence supports an important role for air pollution in the accelerated transmission of COVID-19, as seen in Italy, as well as in Wuhan, China. Lockdown-related reductions in selected air pollution indicators have conversely contributed to reduced transmission of the virus, while high levels of particulate matter or microorganisms in the air as a result of excess air pollution might have initially accelerated COVID-19 transmission and risk or continue to add to COVID-19 related mortality rates. Hence, post-lockdown increases in pollution may again accelerate COVID-19 transmission as well as adding to the burden of COVID-19 morbidity and mortality 34.

Research further implies that COVID-19 is largely transmitted by air and the presence in the air of certain forms of atmospheric particulate matter potentially create a suitable environment for transporting the virus at greater distances than those considered for close contact. Moreover, particulate matter is said to induce lung cell inflammation and this could increase the susceptibility and severity of the COVID-19 related symptoms. In Italian cities for example, retrospective data show particulate matter daily concentrations were higher than the annual average in the months preceding the COVID-19 pandemic 35.

In a systematic review on this issue, it was concluded however, that as a whole, this association, is hard to validate because researchers often use different research methods or do not include confounding factors into their analyses. In addition, to date incidence data are possibly underestimated in all countries and to a lesser extent the same applies to mortality data. For this reason, the cases included in the reviewed studies could not be considered conclusive, but major findings are tentatively consistent as regards the importance contribution of selected air pollutants to trigger the spread of COVID-19. These pollutants also tend to increase lethality 36.

As outlined above, exposure to chronic levels of air pollutants currently constitutes a widespread human and global health risk 35. At the same time, the synthesis of vitamin D in human skin due to sun exposure may be reduced by air pollution causing a possible vitamin D deficiency 35. Prospective and observational studies confirm populations living in different geographic areas may show an increased prevalence of a low vitamin D status in the presence of air pollution 35. According to others, one possible reason for the high rates of vitamin D deficiency is the presence of high levels of air pollution, as air pollution might block those ultraviolet rays that need to be absorbed by the skin and translated into active vitamin D 3738. Ritu et al. 39 similarly note that atmospheric pollution even in metropolitan India, which is otherwise sunny, potentially impacts vitamin D status adversely.  He He et al. 40, similarly note that a vitamin D deficiency is common in sub-tropical locations, such as the Sichuan Basin, which is related to solar radiation and air pollution, rather than climatic conditions. Reasons for concern according to these authors and others are the following: 

High levels of air pollution affect the body’s natural defenses against airborne viruses, increasing the likelihood that people will contract viral diseases, such as COVID-19, especially those who are vitamin D deficient.

Exposure to air pollution increases the risk for many of the chronic diseases that are often linked to vitamin D deficits, and tend to render people more likely to develop serious illnesses requiring intensive care and mechanical ventilation due to COVID-19.

Exposure to air pollution can worsen symptoms of respiratory infections, while increasing the risk of hospitalization and death 414243