Purpose To monitor the effectiveness of the single dose Ad26.COV2.S COVID-19 vaccine among health care workers (HCW) as compared to the general unvaccinated population 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, but currently no vaccine has been rolled out. The recent promising results of the 'ENSEMBLE' trial conducted by Janssen in South Africa, and the availability of a limited amount of vaccine doses, provide the rationale for a cohort study of vaccinated HCWs to inform the larger vaccine rollout.

Study participants Health Care workers age 18 and above working in the South African public and private health care sector (N=500 000) 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 single dose of vaccine at enrolment; to monitor outcomes the DATCOV surveillance system and NICD line list will be reviewed for up to 2 years post-vaccination.

A sub-cohort will be followed up at Months 3 and Month 6. This group will also have outcomes monitored for up to a further 18 months, bringing their total follow-up duration to 2 years post-vaccination.

Study products Ad26.COV2.S by Janssen administered as a single injection

Primary objectives • To assess the effectiveness of Ad26.COV2.S vaccine on severe COVID, hospitalizations and deaths in HCWs as compared with the general unvaccinated population in South Africa.

Secondary objectives • To estimate the incidence of symptomatic SARS CoV-2 infections in HCW

To estimate vaccine uptake among HCWs in South Africa

To monitor the genetic diversity of breakthrough SARS CoV-2 infections

In a subgroup of participants:

To compare serum neutralization and immune responses before and after vaccination

To monitor for asymptomatic infection Exploratory objectives • To establish a link between the national pharmacovigilance system to assist with monitoring safety and any unexpected adverse effects