Sisonke 2 - A COVID-19 Vaccine Boost Open Label Study.

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

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.

Eligibility Criteria:
Inclusion Criteria

* Age 18 and older
* All Sisonke participants
* Received a priming Ad26.SARS.COV.2.S vaccination as part of the Sisonke study at least 6 months prior
* Participants who are pregnant or report breastfeeding at the time of enrolment may be included.
* Willingness and ability to comply with vaccination plan and other study procedures.
* Capable of giving electronic or personal signed informed consent as described in Appendix 5, which includes compliance with the requirements in this protocol.

Exclusion Criteria:

* Participants who have received boosting vaccination through other means.
* Any significant acute or chronic medical condition, situation or circumstance that in the opinion of the PI/designee makes the participant unsuitable for participation in the study, or jeopardises the safety or rights of the participant
* Current participation in any other research studies that would interfere with the objectives of this study. The determination of whether participation in another study would be exclusionary for a given participant will be made by the PI/designee.
* Participants with a history of heparin-induced thrombocytopenia or TTS