Seroprevalence surveys in sub-Saharan Africa: what do they tell us?
Authors: Effua Usuf, Anna Roca
Journal: The Lancet Global Health (2021)

Initial estimates of the potential effect of SARS-CoV-2 virus in sub-Saharan Africa were catastrophic:1Massinga Loembe M Tshangela A Salyer SJ Varma JK Ouma AEO Nkengasong JN COVID-19 in Africa: the spread and response.Nat Med. 2020; 26: 999-1003Crossref PubMed Scopus (92) Google Scholar fortunately, these predictions have not been met in terms of number of cases, deaths, and saturation of health systems. On one hand, since ageing is the main risk factor for COVID-19,2de Lusignan S Dorward J Correa A et al.Risk factors for SARS-CoV-2 among patients in the Oxford Royal College of General Practitioners Research and Surveillance Centre primary care network: a cross-sectional study.Lancet Infect Dis. 2020; 20: 1034-1042Summary Full Text Full Text PDF PubMed Scopus (286) Google Scholar the young population in sub-Saharan Africa partly explains the low number of cases and deaths registered. On the other hand, the limited testing capacity in many sub-Saharan Africa countries, accompanied by stigmatisation, might result in underestimation of the magnitude of transmission. Underestimation of the true burden of COVID-19 has been indicated by seroprevalence studies in several sub-Saharan Africa countries.3Chibwana MG Jere KC Kamng'ona R et al.High SARS-CoV-2 seroprevalence in health care workers but relatively low numbers of deaths in urban Malawi.medRxiv. 2020; (published online Aug 5.) (preprint).https://doi.org/10.1101/2020.07.30.20164970PubMed Google Scholar, 4Uyoga S Adetifa IMO Karanja HK et al.Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Kenyan blood donors.Science. 2021; 371: 79-82Crossref PubMed Scopus (90) Google Scholar, 5Olayanju O Bamidele O Edem F et al.SARS-CoV-2 seropositivity in asymptomatic frontline health workers in Ibadan, Nigeria.Am J Trop Med Hyg. 2021; 104: 91-94Crossref PubMed Google Scholar However, most of these studies were done in specific groups that might not be representative of the general population. In The Lancet Global Health, Lloyd Mulenga and colleagues6Mulenga LB Hines JZ Fwoloshi S et al.Prevalence of SARS-CoV-2 in six districts in Zambia in July, 2020: a cross-sectional cluster sample survey.Lancet Glob Health. 2021; (published online March 9.)https://doi.org/10.1016/S2214-109X(21)00053-XSummary Full Text Full Text PDF Scopus (44) Google Scholar describe the first large population-based seroprevalence survey in the region (covering six districts in Zambia) to determine the extent of SARS-CoV-2 transmission in the country. To complement overall rates of infection, the investigators also collected nasopharyngeal swabs to determine SARS-CoV-2 prevalence by real-time PCR (rtPCR) and thus ongoing infection in the population since the survey was done during the first wave of the epidemic. Two main findings from the study by Mulenga and colleagues contribute to the growing evidence of under-reporting and the mild symptomatology of SARS-CoV-2 infection in sub-Saharan Africa. First, the study indicates that official data on the number of laboratory-confirmed cases are largely underestimating the extent of community transmission. According to the study, only one laboratory-confirmed case was reported for every 92 SARS-CoV-2 infections that occurred in the community.6Mulenga LB Hines JZ Fwoloshi S et al.Prevalence of SARS-CoV-2 in six districts in Zambia in July, 2020: a cross-sectional cluster sample survey.Lancet Glob Health. 2021; (published online March 9.)https://doi.org/10.1016/S2214-109X(21)00053-XSummary Full Text Full Text PDF Scopus (44) Google Scholar Only 2·3% of individuals with a positive rtPCR test were aware of their infection and only 8·2% of individuals with SARS-CoV-2 antibodies were aware that they had been infected because they had not been tested. At the time of the survey (July 4–27, 2020), fewer than 5000 cases had been reported in Zambia, whereas the study data indicated that an estimated 454 708 SARS-CoV-2 infections (95% CI 312 705–596 713) had occurred in the six participating districts between March and July, 2020. Absence of systematic surveillance and testing strategies are possible factors that might have contributed to these differences, which have also been observed in other settings.7Mendelson M Madhi S South Africa's coronavirus testing strategy is broken and not fit for purpose: it's time for a change.S Afr Med J. 2020; 110: 429-431PubMed Google Scholar Widespread misinformation in the community and the stigma associated with COVID-19 are also important factors, which result in avoidance of testing and self-medication.8Sadio AJ Gbeasor-Komlanvi FA Konu RY et al.Assessment of self-medication practices in the context of the COVID-19 outbreak in Togo.BMC Public Health. 2021; 21: 58Crossref PubMed Scopus (31) Google Scholar Second, most of the cases identified by the study were asymptomatic or had mild symptoms. Discrepancies between official numbers and infections measured by the survey can also be explained by this high prevalence of asymptomatic infections, since most testing strategies focus on patients who are symptomatic. This finding is consistent with other studies done in Africa,5Olayanju O Bamidele O Edem F et al.SARS-CoV-2 seropositivity in asymptomatic frontline health workers in Ibadan, Nigeria.Am J Trop Med Hyg. 2021; 104: 91-94Crossref PubMed Google Scholar and is aligned with the community belief of unjustified national and international response to the virus. With time, Zambians became less adherent to prevention measures due to the perception of low personal risk.6Mulenga LB Hines JZ Fwoloshi S et al.Prevalence of SARS-CoV-2 in six districts in Zambia in July, 2020: a cross-sectional cluster sample survey.Lancet Glob Health. 2021; (published online March 9.)https://doi.org/10.1016/S2214-109X(21)00053-XSummary Full Text Full Text PDF Scopus (44) Google Scholar Mulenga and colleagues should be commended for the timeliness of their survey and swift reporting of results. Seroprevalence surveys done during the peak of the pandemic are likely to underestimate the true magnitude of COVID-19 in the community; therefore, inclusion of PCR testing in this study was beneficial to identify both recent (ie, in the past 2–3 weeks) and past infections. If most of infections in the continent are indeed asymptomatic, seroprevalence studies might underestimate the true extent of transmission, since antibody response is proportional to severity of infection.9Long QX Tang XJ Shi QL et al.Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections.Nat Med. 2020; 26: 1200-1204Crossref PubMed Scopus (1599) Google Scholar However, only half of the participants agreed to provide samples for both PCR and ELISA and, therefore, overall infection rate is probably higher than reported in the survey. One limitation of the study design is that it is difficult to estimate the overall transmission at the end of the first wave of COVID-19 infections. Although the majority of cases of COVID-19 have been mild in sub-Saharan Africa, the overall impact of the pandemic is yet to be quantified. Studies on excess mortality in African countries such as Zambia are necessary to measure the overall direct and indirect impact on mortality. Additionally, the prompt response of the Zambian Government, which included the closure of borders, curfews, and school closures, could have additional indirect effects, which should be weighed against the direct effects of the pandemic. The overall impact of the pandemic needs to be monitored while accurate information is shared with the public to encourage communities to comply with the restrictions. As noted by the authors, most Zambians remained susceptible to SARS-CoV-2 infection at the time of their survey, thus the higher number of COVID-19 cases reported in the second wave in that country that began towards the end of December, 2020, is not surprising. A follow-up seroprevalence survey will be useful as the epidemic continues and plans for vaccination advance. In sub-Saharan African settings with laboratory capacity, we advocate for seroprevalence studies whenever possible. Furthermore, combining these studies with excess mortality studies would provide reliable data to support governments to make decisions on how to manage the pandemic and reduce the health and socioeconomic impact. Ultimately, to understand transmission in Zambia and the sub-Saharan African region, asymptomatic surveillance is warranted. We declare no competing interests. Prevalence of SARS-CoV-2 in six districts in Zambia in July, 2020: a cross-sectional cluster sample surveyThe estimated number of SARS-CoV-2 infections was much higher than the number of reported cases in six districts in Zambia. The high rtPCR-positive SARS-CoV-2 prevalence was consistent with observed community transmission during the study period. The low ELISA-positive SARS-CoV-2 prevalence might be associated with mitigation measures instituted after initial cases were reported in March, 2020. Zambia should monitor patterns of SARS-CoV-2 prevalence and promote measures that can reduce transmission. Full-Text PDF Open Access