Sisonke 4 (SHERPA)/ mRNA-1273-P508

South Africa is severely affected by the global COVID-19 pandemic. As part of the Sisonke study, HCWs received a single dose of Ad26.COV2.S vaccine between 17 February and 17 May 2021, and were then offered a second dose between 8 November and 17 December 2021. Immunogenicity studies have since indicated that a heterologous mRNA vaccine boost strategy may elicit stronger neutralizing antibody responses among people who received the Ad26.COV2.S and other vector-based vaccines.The purpose of the study is therefore to evaluate the effectiveness of the heterologous mRNA-1273 (Moderna) boost against COVID-19 infections and severe disease (including deaths) among health care workers (HCWs) who participated in the Sisonke trial and received either a single dose or two doses of Ad26.COV2.S (Janssen, Johnson and Johnson) COVID-19 vaccine as their primary vaccination. Furthermore, the study aims to collect additional safety and immunogenicity data on the heterologous mRNA-1273 boost among Sisonke participants.