Rates of severe acute respiratory syndrome coronavirus 2 infection in a highly exposed South African blood donor cohort, 2022–2023

BACKGROUND AND OBJECTIVES: A cohort of South African repeat blood donors was established to leverage the repeated presentation of blood donors for longitudinal serological surveillance of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and reinfection incidence in a highly exposed population. MATERIALS AND METHODS: The prospective cohort was followed from January 2022 to December 2023, with a total of 7459 blood samples from 2155 donors captured and tested using the Roche anti-nucleocapsid (N) and anti-spike (S) total immunoglobulin antibody (Ab) assays. First infections were identified by anti-N seroconversion and reinfections by anti-N boosting (≥1.5-fold) in longitudinal testing, using a novel method to reduce misclassification from prolonged Ab maturation. Incidence was estimated, and correlates of infection were assessed using multivariable Cox proportional hazards regression. RESULTS: A total of 948 infection events were observed during the study period-280 first infections and 668 reinfections, with up to 4 serial infection events in one donor. Total infection incidence was estimated at 64.2 infections/100 person-years (95% confidence interval [CI]: 60.2-68.4). In multivariable models, quartile of anti-N reactivity was a strong correlate of protection against reinfection with an adjusted hazard ratio of 0.26 (95% CI: 0.18-0.38). CONCLUSION: Longitudinal blood donor serosurveillance allowed quantification of continuing SARS-CoV-2 transmission after most individuals had been previously infected in the 2 years after relaxation of non-pharmaceutical interventions.