Background
Despite the total population of 1.3 billion, Africa stands out as the region least affected by the severe acute respiratory syndrome-Corona-Virus-2 (SARS-CoV-2) and coronavirus disease-2019 (COVID-19) pandemic. As of May 23rd, 2021[
1], the total reported case number had risen to 4,748,581 with 128,213 reported deaths, representing 2.9% and 3.7% of global cases and deaths, respectively. The low number of reported cases and deaths in Africa has been attributed to low testing capacity, younger population, warmer environments, and the successful implementation of control measures [
2]. Also, pre-existing cross-protective immunity due to the four other less pathogenic human coronaviruses (HCoVs) [
3], Bacillus Calmette-Guérin (BCG)-vaccination [
4], or recent history of malaria infection may offer some protection against infection or severe forms of COVID-19[
5].
As of May 21, 2021, Ethiopia has performed over 2,682,758 real-time reverse transcription-polymerase chain reactions (RT-PCR) tests for SARS-CoV-2 and reported 268,901 cases and 4068 deaths since the first case was detected in the country on March 13, 2020. Almost all testing has been done to confirm SARS-CoV-2 infection in suspected cases and contacts, as well as both outbound and inbound travelers. Given the difficulty and cost of RT-PCR-based testing in resource-limited countries like Ethiopia, mildly affected or asymptomatic individuals are not usually screened, and so the number of confirmed SARS-CoV-2 infections is likely vastly underestimated [
6]. In this context, seroprevalence surveys are of the utmost importance to assess the proportion of the population that have already developed antibodies against the virus.
Evidence has shown that healthcare workers (HWs) are at higher risk of acquiring the infection than the general population. This is because their work is likely to require close contact with SARS-CoV-2 infected patients at COVID-19 treatment centers, in emergency rooms and wards, and via virus-contaminated surfaces. If infected, they can pose a significant risk to vulnerable patients and co-workers [
7]. Thus, assessing the seroprevalence of SARS-CoV-2 antibodies among HWs in Ethiopia will help us understand COVID-19 spread among health care facilities and to measure the success of public health interventions. It will also provide an opportunity to compare the disease trajectory in a low-income setting. A report from London, UK suggested that the rate of asymptomatic SARS-CoV-2 infection among HWs reflects general community transmission rather than in-hospital exposure [
8].
Currently, numerous commercial COVID-19 antibody tests, including Enzyme Linked Immunosorbent Assay (ELISA) are available. However, most of them lack validation and evaluation of their sensitivity and specificity in large-scale studies, particularly in Africa, where interpretation of results from these tests is challenging due to pre-existing cross-reactive antibodies induced by other pathogens [
9,
10]. However, in resource-limited countries, including Ethiopia, data generated from validated antibody tests using local clinical samples could provide information valuable in combating COVID-19. Unfortunately, the price and supply of antibody tests are unpredictable. To overcome these challenges, we have developed a relatively cheaper in-house ELISA to detect anti–SARS-CoV-2 Receptor Binding Domain (RBD) immunoglobulin G (IgG) antibodies in serum samples. We then conducted a serosurvey amongst HWs in five public hospitals to estimate the seroprevalence of SARS-CoV-2 in urban Ethiopia using our validated assay. We discussed the implications of our SARS-CoV-2 serosurveillance findings for frontline HWs and the Ethiopian population at large.
Discussion
Interpretation of SARS-CoV-2 serologic test results, except pan Igs Wanti ELISA, has been reported to be very challenging in Africa due to pre-existing cross-reactive antibodies induced by other pathogens such as non-SARS-CoV-2 human coronaviruses and malaria parasites [
9]. Given the rapid decline of anti-SARS-CoV-2 nucleocapsid antibodies as compared to the anti-RBD IgG antibody [
13], we developed and optimized an in-house ELISA that detects anti-SARS-CoV-2 IgG antibodies. Our assay, unlike other commercially available serologic assays, is affordable and has been validated with a large number of Ethiopian sera from both pre-COVID-19 and COVID-19 patients from the same regions. Its sensitivity on convalescent sera from COVID-19 patients confirmed by RT-PCR was found to be as sensitive as the Wantai pan Ig ELISA (100%), and superior to Realy Tech’s IgM/IgG LFA (90%). Also, our in-house assay displayed 97.7% specificity in randomly selected pre-COVID-19 Ethiopian origin sera, which is superior to Realy Tech (92.5%).
Seroprevalence studies provide information about the extent of individuals who had exposure to the virus and help to understand the future course of the pandemic and are key to providing target prevention and control measures in reducing transmission and severe outcomes [
15]. In this study, the overall seroprevalence of SARS-CoV-2 spike RBD IgG antibodies among HWs was 39.6%, ranging from 24.5% in the Hiwot Fana Specialized Hospital, Harar to 48·0% in ALERT Hospital located in the capital city, Addis Ababa. This is not a surprise given Addis Ababa is the epicenter of SARS-CoV-2 transmission in Ethiopia, and SARS-CoV-2 has been introduced 4 months later in Harar. As a result, it is expected that a higher proportion of HWs in hospitals located in Addis Ababa, including ALERT are more frequently exposed to COVID-19 cases than that HWs working in hospitals located in Harar, where fewer number cases and deaths had been reported.
According to our findings, at least 4 in 10 urban Ethiopian HWs had already been exposed to SARS-CoV-2 by February 2021 in Ethiopia. This result contrasts with a serosurvey in asymptomatic individuals from the general population conducted in March 2020 in Addis Ababa (8.8%) [
16] and from the household serosurveys in Jimma (2%) and Addis Ababa (5%) that were conducted during the first wave of the pandemic-i.e., four months after the first COVID-19 case in Ethiopia [
17]. Although this stark seroprevalence difference between our study and these two previous studies might be explained by differences in the types of assays employed, lack of personal protective equipment (PPE) and/or cohort types. The most plausible explanation is that the sera for the present serosurveillance study had been collected after the first wave of the pandemic in Ethiopia, between March 2020 and February 2021.
While the high seroprevalence rates observed among the different geographically located hospitals are approaching those of high-incidence countries like Brazil [
18], they are in agreement with several other SARS-CoV-2 seroprevalence studies from sub-Saharan Africa that, like Ethiopia, have reported much lower rates of RT-PCR confirmed cases and deaths. For example, higher anti-SARS-CoV-2 antibody seroprevalence has been reported in South Sudan (30–60.6%) [
19], Democratic Republic of Congo (8–36%) [
20] and Nigeria (25–45%) [
21] depending on the population sampled and the serological test used. Taken together these studies indicate that SARS-CoV-2 has spread widely in sub-Saharan Africa [
22]. However, the majority (74.0%) of our study participants never had any symptoms compatible with COVID-19, suggesting the occurrence of significant burden of asymptomatic infections and its transmissions in the country, which is now, being reflected in the trend of increasing RT-PCR positivity since January 2021 (
https://covid19.who.int/region/afro/country/et). The higher proportion of younger HWs (mean age of 34 years), and the fewer participants with comorbidities (6.7%) may have contributed to the observed high burden of asymptomatic infection among the studied HWs. Malaria, BCG-vaccination, warmer environment, and high prevalence of pre-existing cross-reactivate against HCoVs may have also contributed[
3].
A report from Spain showed a higher (38.3%) seroprevalence of SARS-CoV-2 among HWs [
23]. This is comparable with the present report from Ethiopia, where there were a relatively fewer severe cases and deaths (
https://covid19.who.int/region/afro/country/et). Similarly, higher seroprevalence among frontline HWs has been reported in other sub-Saharan African countries such as in Malawi [
24]. These findings and ours highlight the importance of asymptomatic infections in the African countries. Interestingly, we found no seroprevalence differences between healthcare occupations including administrative staff. The lack of a dramatic difference among frontline HWs and administrators may be a reflection of the frontline administrative staff are also at high risk and are poorly protected, or may suggest the level of virus transmission in the general population at large as previously observed in UK [
8]. Nevertheless, further well-designed investigations are required to implement occupation-specific public health strategies in healthcare facilities.
In the present study, a history of previous close contact with a suspected or confirmed COVID-19 case was found to be strongly associated with seropositivity; however, this finding contradicts the observed similar seropositivity among front line HWs and administrators. Similar odds of seropositivity among males and females were also found although several studies elsewhere reported higher odds of seropositivity in males [
25]. A similar contradictory finding with no seroprevalence differences by sex was reported in the Spanish general population [
23].
Our study has several strengths. These include its use of an in-house developed assay which we optimized to significantly minimize false positive responses by validating it with both pre-pandemic and pandemic samples of Ethiopian origin. Most importantly, the study involved a relatively large sample size from five hospitals located in different geographical locations, providing much needed information about the COVID-19 pandemic in sub-Saharan Africa.
Despite its strengths, this study has several limitations. First, we did not employ a random sampling technique. Instead, we invited all hospital staff to take part in the study on a voluntary basis. Hence selection bias might have affected our results. For example, although we invited all staff to reach the desired sample size per cadre, some staff who had exposure to SARS-CoV-2 might not had been enrolled in our study in fear/misconception that being seropositive would lead them to undergo quarantine. If this were the case, it would have led to underreporting of seropositive cases. Second, recall bias might have affected the responses to the history of symptoms compatible with COVID-19, and close contact with a confirmed COVID-19 case, and thereby contributed to the absence of a strong correlation between seropositivity and these covariates, albeit having close contact with COVID-19 case. Third, although the sensitivity of our assay was 100% in convalescent serum samples from COVID-19 patients, its specificity is not 100%. Our assay’s specificity, determined by using pre-COVID-19 pandemic sera/plasmata those were collected before December 2019 was found to be 97.7% and hence could lead to overestimating of HWs tested positive to anti- SARS-CoV-2 RBD IgG antibodies. For example, for every 1000 people tested by our assay, 23 HWs who never had SARS-CoV-2 infection might have been incorrectly identified as they had antibodies specific to SARS-CoV-2. However, even this slight overestimation of the apparent seroprevalence associated with the assay specificity is likely to be matched by the proportion of study participants who might be infected and yet not produce humoral immune responses at the time of blood sample collection.
In conclusion, we developed an in-house IgG ELISA that meets the WHO requirements to be utilized for SARS-CoV-2 serosurveillance studies. This seroprevalence study revealed a remarkably high seroprevalence (40–48%) of SARS-CoV-2 among HWs in the five public hospitals; with slight differences amongst hospitals, except Hiwot Fana Specialized Hospital in which relatively lowest (24.5%) seroprevalence was found. We found no seroprevalence rate differences between front line HWs and administrative staff, indicating the observed high seroprevalence of SARS-CoV-2 might also be a reflection of the community transmission. Taken together these findings suggest extensive cryptic circulation (asymptomatic transmission) of SARS-CoV-2 in Ethiopia. Whether the detected anti-SARS-CoV-2 antibodies can persist adequately and confer protection from subsequent infections to those HWs who had or had not received COVID-19 vaccine will require further immunological investigation.
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