The study analyzed data from a randomized efficacy trial of the ChAdOx1 nCoV-19 (AZD1222) vaccine in the United Kingdom to determine the antibody levels associated with protection against SARS-CoV-2. Higher levels of binding and neutralizing antibodies at 28 days after the second dose were correlated with a reduced risk of symptomatic infection. The vaccine achieved an efficacy of 80% against symptomatic infection with the Alpha variant, with specific antibody levels of 264 binding antibody units (BAU)/ml and 506 BAU/ml for anti-spike and anti-RBD antibodies, respectively. These levels were also associated with pseudovirus and live-virus neutralization titers of 26 IU/ml and 247 normalized neutralization titers (N50), respectively. However, these immune markers were not correlated with asymptomatic infections. The findings can be used to bridge to new populations using validated assays and extrapolate efficacy estimates for new COVID-19 vaccines.The study analyzed data from a randomized efficacy trial of the ChAdOx1 nCoV-19 (AZD1222) vaccine in the United Kingdom to determine the antibody levels associated with protection against SARS-CoV-2. Higher levels of binding and neutralizing antibodies at 28 days after the second dose were correlated with a reduced risk of symptomatic infection. The vaccine achieved an efficacy of 80% against symptomatic infection with the Alpha variant, with specific antibody levels of 264 binding antibody units (BAU)/ml and 506 BAU/ml for anti-spike and anti-RBD antibodies, respectively. These levels were also associated with pseudovirus and live-virus neutralization titers of 26 IU/ml and 247 normalized neutralization titers (N50), respectively. However, these immune markers were not correlated with asymptomatic infections. The findings can be used to bridge to new populations using validated assays and extrapolate efficacy estimates for new COVID-19 vaccines.