High infectious disease burden as a basis for the observed high frequency of asymptomatic SARS-CoV-2 infections in sub-Saharan Africa

Abstract

Following the coronavirus outbreaks described as severe acute respiratory syndrome (SARS) in 2003 and the Middle East respiratory syndrome (MERS) in 2012, the world has again been challenged by yet another corona virus, named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 infections were first detected in a Chinese Province in December 2019 and then declared a pandemic by the World Health Organization in March 2020. An infection caused by SARS-CoV-2 may result in asymptomatic, uncomplicated or fatal coronavirus disease 2019 (COVID-19). Fatal disease has been linked with the uncontrolled “cytokine storm” manifesting with complications mostly in people with underlying cardiovascular and pulmonary disease conditions. The severity of COVID-19 disease and the associated mortality has been disproportionately lower in terms of number of cases and deaths in Africa and also Asia in comparison to Europe and North America. Also, persons of colour residing in Europe and North America have been identified as a highly susceptible population due to a combination of several socioeconomic factors and poor access to quality healthcare. Interestingly, this has not been the case in sub-Saharan Africa where majority of the population are even more deprived of the aforementioned factors. On the contrary, sub-Saharan Africa has recorded the lowest levels of mortality and morbidity associated with the disease, and an overwhelming proportion of infections are asymptomatic. Whilst it can be argued that these lower number of cases in Africa may be due to challenges associated with the diagnosis of the disease such as lack of trained personnel and infrastructure, the number of persons who get infected and develop symptoms is proportionally lower than those who are asymptomatic, including asymptomatic cases that are never diagnosed. This review discusses the most probable reasons for the significantly fewer cases of severe COVID-19 disease and deaths in sub-Saharan Africa.

Prevalence of SARS-CoV-2 and co-infection with malaria during the first wave of the pandemic (the Burkina Faso case)

“…COVID-19 transmission in Africa has been marked by relatively fewer infections, mostly asymptomatic, and lower death rate compared to developed countries. A younger population structure and a variety of socio-ecological factors (i.e., warm weather, low population density and mobility and trained immunity by previous high burden infectious diseases), as well as early public health measures taken by governments (i.e., early lockdown sub-Saharan countries) ( 20 ), have been suggested to explain such lower incidence ( 21 ). Surprisingly, the pandemic has been more pronounced in a few countries (e.g., South Africa, Morocco and Tunisia) suggesting country-specific drivers of SARS-CoV-2 spread and morbidity, but this could also be largely explained by a low and unequal testing capacity.…”

“…Previous studies have also indicated that malaria-induced immunomodulation could be protective against respiratory viruses, reducing pulmonary inflammatory response inflammation ( 47 , 48 ). Alternatively, the higher burden of infectious diseases in sub-Saharan Africa has been suggested to mediate the asymptomatic SARS-CoV-2 infections by induction of immunological tolerance or trained immunity to other infections such as tuberculosis, other human coronaviruses and even Bacillus Calmette-Guérin (BCG) vaccination ( 21 , 46 ). In addition, other studies have reported that bacterial, fungal and viral co-infections with SARS-CoV-2 are also uncommon, however when present, they may cause a worse outcome ( 49 – 51 ).…”

The COVID-19 pandemic in the African continent

“…Aside these pathology-associated immune responses, specific adaptive responses to viral antigens have been demonstrated, although the extent of protection attributable to these responses remains to be fully established. Aside from immunity, other factors such as age, gender and comorbidities [ 116 ], race [ 117 ], socioeconomic status [ 118 ], and infectious disease burden [ 119 ] have variously been identified as important factors that predispose infected persons to either severe, fatal disease, or asymptomatic/mild disease. The cell-mediated immune response is the principal and effective immune response against viral infection.…”

An exploration of the political, social, economic and cultural factors affecting how different global regions initially reacted to the COVID-19 pandemic

“…Spending time outdoors with skin exposure to sunlight also enhances their vitamin D production which is known to boost host immunity [ 120 , 121 ], which may possibly reduce the rate of successful infection and severe disease with COVID-19 [ 122 , 123 ]—as has been indicated more definitively for influenza and other respiratory viruses [ 124 , 125 ]. Others have hypothesized that the chronic exposure of the African population to many pathogens could have induced some tolerance to inflammation; together with the widespread use of live attenuated vaccines, like the Bacillus Calmette-Guérin (BCG) vaccine, which may elicit a bystander protective effect [ 126 ].…”