Prevalence of anti-SARS-CoV-2 antibodies among blood donors in South Africa during the period January-May 2021
Authors: Vermeulen Marion , Mhlanga Laurette , Sykes Wendy , Coleman Charl , Pietersen Nadia , Cable Russell , Swanevelder Ronel , Glatt Tanya Nadia , Grebe Eduard , Welte Alex , Berg Karin van den 
Published: August 2021

Abstract
Background: Population-level estimates of the prevalence of anti-SARS-CoV-2 antibody positivity (seroprevalence) are crucial epidemiological indicators for tracking the Covid-19 epidemic. Such data are in short supply, both internationally and in South Africa. The South African blood services (the South African National Blood Service, SANBS and the Western Cape Blood Service, WCBS) are coordinating nationwide surveillance of blood donors. Methods: Leveraging existing arrangements, SANBS human research ethics committee permission was obtained to test blood donations collected on predefined days (in January and May 2021) for anti-SARS-CoV-2 antibodies, using the Roche Elecsys Anti-SARS-CoV-2 assay on the cobas e411 and e801 platforms currently available in the blood services’ donation testing laboratories. Using standard methods, prevalence analysis was done by province, age, time, sex and race. Results: We report on data from 16762 donations. Prevalence varied substantially across race groups and between provinces, with seroprevalence among Black donors consistently several times higher than among White donors, with the other main population groups (Coloured and Asian) not well represented in all provinces. There is no clear evidence that seroprevalence among donors varies by age or sex. The weighted national estimate of prevalence (in the core age range 15-69 years) is 47.4% (95% CI 46.2-48.6). From January to May, we noted a slight but statistically insignificant increase in seroprevalence in those provinces (Gauteng and Free State) where sufficient data were available to make such an estimate. Conclusions: Our study demonstrates substantial differences in dissemination of SARS-CoV-2 infection between different race groups and provinces, in patterns consistent with known differences in historically entrenched socio-economic status and housing conditions. As has been seen in other contexts, even such high seroprevalence does not guarantee population-level immunity against new outbreaks, as evidenced by a substantial third wave that has emerged almost contemporaneously with the end of sampling in this study. The relative importance of various contributions to this resurgence (notably viral evolution, waning of antibody neutralization efficacy, and infection control fatigue) are unclear. Despite its limitations, notably a ‘healthy donor’ effect and the possible waning of detectable antibodies over the time scale of the COVID-19 pandemic, it seems plausible that these estimates are reasonably generalisable to actual population level antiSARS-CoV-2 seroprevalence. The interpretation of occasional seroprevalence surveys as a proxy for total attack rates, over the ever-lengthening pandemic time scale is likely to become ever more complex. More frequent sampling, including linked repeat observations of frequent donors, could substantially improve the utility of blood donor surveillance.