Purpose To evaluate the effectiveness and safety of the single dose Ad26.COV2.S (Jansen) COVID-19 vaccine plus a homologous boost with Ad26.COV2.S (Janssen) COVID-19 vaccine among Sisonke participants as compared to unboosted Sisonke participants~In addition the investigators will continue to evaluate VE of the Sisonke Boost compared to:~i) Vaccinated populations pre boosts ii) Unvaccinated populations in South Africa.~Study design Open-label, single-arm phase 3B vaccine implementation study~Rationale South Africa is severely affected by the global COVID-19 epidemic, and following the initial prime vaccination among HCWs in the first 4 months of 2021. New data has demonstrated the safety and effectiveness of a booster dose given two months or more after the initial Ad26.COV2.S. This provides the rationale and feasibility for the evaluation of a homologous booster vaccine dose to the cohort of vaccinated Sisonke participants to inform the larger vaccine rollout.~Study participants Sisonke participants age 18 and over working in the South African public and private health care sector (approx N=500 000) who were enrolled in Sisonke and have not subsequently had a further booster vaccine dose.~Study sites Department of Health Vaccine Administration Sites across South Africa supported by the Sisonke (Together) (VAC31518COV3012) Trial Research Site Investigators and Study Staff~Study duration Participants will receive a homologous Ad26.COV2.S (Janssen) booster dose of vaccine at least 6 months post the prime vaccination. The investigators will monitor outcomes utilising the DATCOV surveillance system and NHLS/NICD SARS COV-2 testing databases for up to 2 years post initial vaccination.~Study products Ad26.COV2.S by Janssen administered as a single dose followed by a single booster injection.~Primary objectives • To assess the effectiveness of Ad26.COV2.S vaccine as a homologous boost on severe COVID, hospitalizations and deaths in Sisonke participants as compared with the unboosted Sisonke populations.~Secondary objectives To assess the effectiveness of Ad26.COV2.S vaccine as a homologous boost on severe COVID, hospitalizations and deaths in Sisonke participants as compared vaccinated and unvaccinated populations of essential workers in South Africa.~To estimate the incidence of symptomatic SARS CoV-2 infections in Sisonke participants following a boost compared with the unboosted Sisonke populations and general vaccinated and unvaccinated population in South Africa~To estimate booster dose uptake among Sisonke participants in South Africa~To monitor the genetic diversity of breakthrough SARS CoV-2 infections.~To monitor safety in the case of homologous boosts in Sisonke participants.
To evaluate the effectiveness of the single dose Ad26.COV2.S COVID-19 vaccine plus a homologous boostwith Ad26.COV2.S COVID-19 vaccine among Sisonke participants as compared to unboosted Sisonkeparticipants; vaccinated; and unvaccinated populations in South Africa