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