Human coronaviruses are common causative agents of respiratory infections with several subtypes being prevalent in many parts of the world. Coronaviruses are a complex group of viruses of the subfamily Coronavirinae in the family Coronaviridae of the order Nidovirales (1, 2). This subfamily has four genera: Alphacoronaviruses, Betacoronaviruses, Gammacoronaviruses and Deltacoronaviruses, of which Alphacoronaviruses (HCoV-229E and HCoV-NL63) and Betacoronaviruses(HCoV-HKU1, SARS-CoV, HCoV-OC43 and MERS-CoV) infect humans (3, 4). They are enveloped with a linear, non-segmented, positive sense, single-stranded RNA genome ranging between 27 kb to 32 kb which shows that they are the largest among the RNA viruses (5, 6). Although they are phenotypically and genotypically diverse, they possess a common genomic organization with the replicase gene occupying two thirds of 5’ end of the genome in which two overlapping large open reading frames, ORF1a and ORF1b are found (5, 7). The ORF1b is the most highly conserved part of the genome encoding for conserved roles such as polymerase and helicase activities (8, 9). The remaining one third of the genome at the 3’ end carries genes that encode for a set of structural proteins in the order 5’-spike (S) to envelope (E) to membrane (M) and lastly nucleocapsid (N) (5). The Nucleocapsid protein binds to the genome and elicits a humoral immune response because it contains several linear epitopes (10) while the spike protein which is the largest, forms oligomonomers on the virus surface.
Human coronaviruses were first discovered in the 1960s as causative agents of self-limited upper respiratory tract infections and until 2002, they were known to cause mild infections but this changed with the emergence of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) (11, 12). To date there are seven known Human coronaviruses (HCoVs) that have been identified, these are HCoV-229E, HCoV-NL63, HCoV-OC43, HCoV-HKU1, SARS-CoV, MERS-CoV and the newest discovered, SARS-CoV-2 (4, 13). The first four HCoVs have been well known to be of worldwide distribution causing approximately 33.3% of human common cold infections (14). However in some cases, these can cause severe illness in the elderly, children and all immunocompromised persons and patients especially those with underlying infections medical conditions like diabetes, hypertension, tuberculosis and AIDs (15, 16). SARS-CoV suddenly emerged in Guangdong Province of China in 2002 causing severe pneumonia characterized by fever, headache and cough but later develops into life threatening respiratory failure and distress (17). The first cases to be diagnosed with SARS were food handlers and workers in wet live-animals’ markets, but soon the disease spread to several countries in a short period infecting more than 8000 people with a mortality rate of 10–11% (18–20). SARS-CoV is believed to have been spilled over to human populations from civets that are intermediate host for SARS-CoV since it has been traced back to bats as their natural hosts (21). After ten years, in 2012, MERS-CoV appeared in Saudi Arabia causing severe human respiratory disease with clinical presentation similar to SARS-CoV but with a higher fatality rate of 35% (22, 23). MERS-CoV found its way into human population from bats via dromedary camels but unlike SARS-CoV, cases of MERS-CoV infections have been restricted to the Arabian Peninsula and cases outside this Peninsula like the outbreak that occurred in South Korea, the index patient had traveled to Saudi Arabia and United Arabs Emirates thus tracing the source of infection back to the Arabian Peninsula (24, 25). Currently the world is facing a pandemic caused by a novel human coronavirus that started from the Hubei Province in China and now has spread the whole world, this novel coronavirus that had been previous named 2019-nCoV is now known as SARS-CoV-2 due to its similarity to the symptoms induced by SARS-CoV(26–28). Initial studies of SARS-CoV-2, linked the infection to a wet wild animal market in Wuhan but genetic analysis have shown that it is similar to a coronavirus that had been isolated from bats but transmitted to human through a yet to be confirmed intermediate host (29). Nevertheless, studies have showed that human to human transmission through droplets and direct contact has been the most important mode of transmission to regions outside Hubei especially by asymptomatic carriers traveling from one area to another(30, 31).
Seroprevalence studies are important in understanding the prevalence of subclinical human coronavirus infections and the population’s herd immunity against these viruses. The Seroprevalence of human coronaviruses varies greatly among studies because of the different antigens, methodologies used, age and other demographic characteristics of the population studied (32). The Seroprevalence estimates for Human coronaviruses range from 5–30% of all respiratory infections with up to 21.6% of the general population having serum antibodies (33, 34). In a previous study to estimate the exposure level of individuals to the circulating coronaviruses in the U.S. Metropolitan population, established 91.3%, 59.2%, 91.8% and 90.8% seropositive for HCoV 229E, HKU1, NL63 and OC43 respectively which is similar to studies that showed 99%, 91%, 98% and 100% seropositive for the respective human coronaviruses (32, 35). These findings are consistent with the cross-sectional and longitudinal seroepidemiological studies that have showed that large proportions of children and adults have serum antibodies to these four non-SARS human coronavirus strains(7). The Seroprevalences of the zoonotic human coronaviruses (SARS, MERS and SARS-2) have been reported to be below 5% in humans especially among those who do not come into contact with the intermediate hosts for these viruses (36–39). Such results suggest that unknown asymptomatic and subclinical infections or unrecognized cases might exist in the general population that can underscore the role of human to human transmission.
Previous studies have also shown that Enzyme-linked immunosorbent assays are useful in diagnosis of HCoV infections with the biggest challenge being cross reactive antibodies that can give rise to false positives especially when the study aims at detecting a specific virus strain. Several studies have made it clear that cross reactivity on serological testing occurs especially when the target is the nucleocapsid proteins (40, 41).The nucleocapsid protein has been recognized as an important target in the development of human coronavirus diagnostics because it induces a good antibody response (42). However, this nucleocapsid protein has a highly conserved region that occurs in the N-terminal portion which induces cross reactivity antibodies among human coronaviruses (34, 43, 44). Whereas this cross reactivity is a setback, diagnostic kits that detect antibodies to the nucleoprotein can have a qualitative screening role during testing of samples. Compared to the nucleocapsid protein, the spike protein contains multiple conformational epitopes that are major inducers of neutralizing antibodies. These antibodies are more specific and more recommended in the confirmatory testing of a particular type of human coronavirus because the spike protein has the least sequence conservation among human coronavirus proteins(33, 45, 46).
Human coronavirus infections are commonly diagnosed by polymerase reaction using cDNA synthesized from RNA extracts from respiratory tract samples. However, for the establishment exposure rates among the population, seroepidemiological studies offer an important avenue for painting a picture on HCoV infections. Although an increasing threat of zoonosis and emerging pandemics caused by HCoVs is now more obvious than ever, there is little known about their seroprevalence in most developing countries especially Uganda. Here we report the seroprevalence of human coronavirus antibodies in hospital-based surveillance sentinel sites in Uganda before the global pandemic of SARS-CoV-2.