The South African National Blood Service Randburg, Gauteng, South Africa
Background/Case Studies: Confirmed Covid diagnoses are inevitably a very substantial undercount of the total number of infections. Blood donors can efficiently provide meaningfully representative seroprevalence estimates, which, depending on epidemic stage and availability of ‘excess mortality’ estimates, can be leveraged into reasonable estimates of total infection counts and infection fatality rate (IFR).
Study
Design/Methods: Blood donors who donated on prespecified days after each of three epidemic waves (beta, delta and first omicron waves), were tested for anti-SARS-CoV-2 nucleocapsid antibodies using the Roche Elecsys anti-SARS-CoV-2 total immunoglobulin assay. Prevalence of antibodies was estimated by age, sex, race and province and compared to official case reporting. Except for the post omicron sampling, on which testing for both anti-nucleocapsid and anti-spike antibodies was done, testing was just for anti-nucleocapsid protein antibodies. Province and age group-specific IFRs were estimated using external excess mortality estimates.
Results/Findings: The overall nationally weighted anti-nucleocapsid seroprevalence estimates 1) after the beta wave, 2) after the delta wave, and 3) after the first omicron wave, were 47%, 71% and 87%, respectively. There was no discernible variation by age and sex, but there were statistically and epidemiologically significant differences by province (except at the latest time point) and race. At the first time point there was on average a 13 fold higher sero-prevalence than confirmed case counts . Overall, age dependent IFR roughly doubled for every ten years of age increase, over six decades of age. Conclusions: Large discrepancies were found between seroprevalence and confirmed case counts, particularly in the less infrastructurally developed provinces. Given historical inequities which impact housing and working conditions, it is unsurprising that particularly high seroprevalence rates were found among some donors even in the earliest round of surveillance. Our IFR estimates (the first in our country) were useful in refining the previously largely evidence free disagreements about the severity of the epidemic in the country. Given the low marginal cost, blood donor-based sero-surveys provided a valuable and efficient way to provide near real-time monitoring of the ongoing SARS-CoV-2 outbreak.
Importance of research: This research highlights donor based sampling as a useful tool to public health faculties in understanding the pandemic it was also the first credible estimation of IFR in our country, which we interpret, as credible estimates of deaths due to SARS-CoV-2 infection. This estimated IFR shows the same exponential, escalation with age, over the full age range, as has been seen in other contexts, with much the same overall scale
