This background document was prepared by the Strategic Advisory Group of Experts (SAGE) on Immunization Working Group on COVID-19 Vaccines to inform the discussions of SAGE at its 15 March 2021 meeting, which resulted in the issuance of the WHO interim recommendations for use of the Ad26.COV2.S (COVID-19) vaccine. Both the recommendations and the background document are available on the SAGE COVID-19 webpage: https://www.who.int/groups/strategic-advisory-group-of-experts-on-immunization/covid-19-materials. Declarations of interests were collected from all external contributors and assessed for any conflicts of interest. Summaries of the reported interests can be found on the SAGE meeting webpage and SAGE Covid-19 Working Group webpage. Context The Janssen vaccine is a recombinant vector vaccine that uses a human adenovirus to express the SARS-CoV-2 spike protein and is based on the Ad26 vector platform. The adenoviruses are a group of viruses that cause infections in the respiratory and gastrointestinal tracts; the adenovirus vector used in the experimental vaccine has been modified, so that it can no longer replicate in humans and cause illness. In developing the vaccine, Janssen employed the same vector used in the first dose of its prime–boost vaccine regimen against Ebola virus disease (Ad26 ZEBOV and MVN-BN-Filo). As of 31 December 2020, Ad26-based vaccines have been used to vaccinate 193 831 participants in clinical studies and vaccination programmes. These more than 193 000 participants included people from different age groups (elderly, adults, children and infants), individuals positive for human immunodeficiency virus (HIV), and pregnant and breastfeeding women, and the data show a favourable safety profile. Ad26-based vaccines elicit strong humoral immune responses with both neutralizing activity and non-neutralizing antibody functionalities, and cellular immune responses involving both CD8+ T cells and CD4+ T-cells, the latter with a predominantly Th1 phenotype, irrespective of the transgene encoded immunogens (1-4). Overall, these vaccines have been shown to have an acceptable clinical safety profile to date.Data taken into consideration are those included in the US Food and Drug Administration Vaccines and Related Biological Products Advisory Committee (VRBPAC) meeting documentation, available under the following links: www.fda.gov/media/146217/download and www.fda.gov/media/146218/download.