ABSTRACT
Background: The Sisonke Phase IIIB open-label implementation study vaccinated health care workers (HCWs) with the single dose Ad26.COV2.S vaccine during two phases of the South African Covid-19 epidemic, dominated first by the Beta followed by the Delta variant of concern. Methods: HCWs were vaccinated over 3 months (17 February-17 May 2021). Safety was monitored by self-reporting, facility reporting and linkage to national databases. Vaccine effectiveness (VE) against Covid-19 related hospitalisation, hospitalisation requiring critical or intensive care and death, ascertained 28 days or more post vaccination was assessed up until 17 July 2021. Nested sub-cohorts (A and B) from two national medical schemes were evaluated to assess VE using a matched retrospective cohort design. Results: Over the 3-month period, 477234 HCWs were vaccinated in 122 vaccination sites across South Africa. VE derived from the sub-cohorts comprising 215 813 HCWs was 83% (95% CI 75-89) to prevent Covid-19 deaths, 75% (95% CI 69-82) to prevent hospital admissions requiring critical or intensive care and 67% (95% CI 62-71) to prevent Covid-19 related hospitalisations. The VE was maintained in older HCWs and those with comorbidities including HIV infection. VE remained consistent throughout the Beta and Delta dominant phases of the study. 10279 adverse events were reported and 139 (1.4%) were serious, including two cases of thrombosis with thrombocytopenia syndrome and four cases of Guillain-Barré syndrome who recovered. Conclusion: The single dose Ad26.COV2.S was safe and effective against severe Covid-19 disease and death post-vaccination, and against both Beta and Delta variants providing real-world evidence for its use globally.