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COVID 19: Severity and Virulence Factors in Different Climates

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COVID 19: Severity and Virulence Factors in Different Climates

Written by

Ndagana Bondira Olamide B.Tech(hons), M.Sc. (Medical Microbiology).



COVID-19 disease is a highly pathogenic and transmittable viral infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which emerged in Wuhan, China and spread around the world. Because of the exponential growth of the infection, on March 11, COVID-19 was officially declared a pandemic by the World Health Organization (WHO).  Genomic analysis reveals that SARS-CoV-2 is phylogenetically related to severe acute respiratory syndrome-like (SARS-like) bat viruses; therefore bats could be the possible primary reservoir. The immediate source of origin and transfer to humans is not known, however, the rapid human to human transfer has been confirmed widely. The virus can be transmitted at a longer distance in free air, but in droplet form it can only spread at distance of 3ft. or 1meter from the infected person to susceptible population.

COVID-19 outbreak has triggered a global crisis that is challenging governments, health systems and the scientific community worldwide with a central question whether climatic factors modulate its progression. Previous attempts to assess the impact of climatic parameters have yielded ambiguous results, either because of geographically or temporally restricted data, or because of comparison of infection rates across countries which were measured differently depending on local screening strategies. In March 2020, the spread of COVID-19 dramatically increased the number of countries recording deaths, providing an opportunity to use mortality rate, which is measured more homogeneously across countries than infection rates, as a descriptor of the COVID-19 outbreak over a large latitudinal range. The debated question about COVID-19 being affected by season-related climatic factors, as other seasonal viruses is of utmost importance, as the possible climatic dependency of COVID-19 could help decision-makers to adopt the most suitable strategy to control the disease in the near future.

Virulence is described as the ability of an organism to infect the host and cause a disease and virulence factors are substances that assist the organism colonize the host at the cellular level. These factors allow it to replicate, modify or take over host defenses and spread within the host and are toxic to the host. They determine whether infection occurs and how severe the resulting viral disease symptoms are. Viruses often require receptor proteins on host cells to which they specifically bind. Typically, these host cell proteins are endocytosed and the bound virus then enters the host cell.

The COVID-19 virus is highly virulent and its ability to cause serious disease inversely corresponds to the person’s ability to produce T-cells which declines linearly with age. Chronic diseases, which have been recognized as virulence factors for severe COVID-19, are often comorbid with protein-energy malnutrition (disease-related malnutrition), which is known to impair immune cell activation thus allowing longer viral persistence and increased trafficking of inflammatory cells to lungs. Pathophysiology and virulence mechanisms of SARS-CoV-2 have links to the function of the non-structural proteins and structural proteins. These non-structural proteins are able to block the host innate immune response facilitating and consolidating an infection. Serious heart conditions, diabetes, and also obesity contributes for a severe outcome of COVID-19 disease.  The protein envelope as a major virulence factor also plays a crucial role promoting viral assembly and release.

Currently, COVID-19 virulence can be determined in relation to type of race or region that is more susceptible to COVID-19 agent, level of melanocytes and epidermal keratinocytes, co-morbidity of virus in the population, its level of chemical toxin injected in the susceptible host, physical trauma caused on lining of the lung, mode of nutrition and environmental factors of the susceptible population health. The current level of resistance in the African countries against COVID-19 may be associated with high quality of melanocytes and epidermal keratinocytes for absorption of vitamin D. African nutrition and tropical African climate that replenishes vitamin D and C have great impact to black race long life immunity that mitigate those population from developing severe symptoms of covid-19 than in the countries like Canada, USA and China.

Warm air temperatures, intense ultraviolet light, or low relative humidity may act by reducing the transmission of the SARS-CoV-2 virus or may directly or indirectly inactivate this virus.

Results from previous studies indicate that, under laboratory conditions, the SARS-Cov-2 virus survives less than 4 days at 37°C, while at 4°C it may remain stable for more than 14 days, offering one possible explanation to the relation between air temperature and the COVID-19 mortality rate. Air temperature and Ultraviolet index are related climatic variables that play a substantial role in explaining the COVID-19 mortality rate. A higher value in one or other of the variables is associated with lower mortality rates.


The world is facing an unprecedented worldwide crisis, forcing decision makers to move forward into the web of unknowns. Understanding how climatic factors have been instrumental to the severity of the COVID-19 outbreak until now could strengthen the projections of its future spread.


Since there are no vaccines or specific antiviral treatments for COVID-19, there are measures to prevent infection which include frequent hand washing, maintaining physical distance from others (especially from those with symptoms), quarantine (for those with symptoms), covering coughs, use of alcohol based sanitizers, keeping unwashed hands away from the face and the use of nose mask to minimize the risk of transmissions.


COVID-19 has high infection and virulence in the European population, its virulence remains low specifically in tropical Africa which is closely associated with high passive immunity, nutrition and tropical climate. Further investigations will help in understanding better why case fatality in these European population compared to the African population is higher.


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