D8110C00001 is a Phase III randomized, double-blind, placebo-controlled multicenter study assessing the safety, efficacy, and immunogenicity of AZD1222 compared to saline placebo for the prevention of COVID-19. Participants will be adults >= 18 years of age who are healthy or have medically-stable chronic diseases, and are at increased risk for SARS-CoV-2 acquisition and COVID-19. Approximately 30 000 participants will be randomized in a 2:1 ratio to receive 2 IM doses of either 5 × 10^10 vp
(nominal, +- 1.5 × 10^10 vp) AZD1222 (n = approximately 20 000) or saline placebo
(n = approximately 10 000) 4 weeks apart, on Days 1 and 29. Randomization will be stratified by age (>= 18 and < 65 years, and >= 65 years), with at least 25% of participants to be enrolled in the older age stratum.

Participants who present with at least one of the qualifying symptoms listed below through Day 360 will be assessed for COVID-19. With the exception of fever, shortness of breath, or difficulty breathing, the symptom must be present for 2 or more days. Participants with a COVID-19 qualifying symptom(s) will be tested for SARS-CoV-2, and if positive will complete illness visit assessments, as presented in Table 4. See Section 8.1 for details on COVID-19 assessments.