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

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