In the Parent study, the Phase I Sentinel Safety Cohort will assess the safety of AZD5156 in healthy adults and the Phase III Main Cohort will assess the safety, efficacy, PK, and neutralizing activity of two doses of AZD3152 compared with two doses of comparator given at a 6-month interval in adults and adolescents 12 years of age or older (weighing at least 40 kg) with conditions causing immune impairment, who are less likely to mount an adequate protective immune response after vaccination and thus are at higher risk of developing severe COVID-19 in 18 countries. Sub-study: This Phase II sub-study of SUPERNOVA is operating in the USA only, and it will assess the safety, PK, and predicted neutralizing activity of AZD3152 in adults 18 years of age or older (weighing at least 40 kg) with conditions causing immune impairment who are less likely to mount an adequate protective immune response after vaccination as well as individuals who are immunocompetent (including healthy participants) with all degrees of SARS-CoV-2 infection risk.