Correlates of protection against symptomatic and asymptomatic SARS-CoV-2 infection
Authors: Feng Shuo , Phillips Daniel J. , White Thomas , Sayal Homesh , Aley Parvinder K. , Bibi Sagida , Dold Christina , Fuskova Michelle , Gilbert Sarah C. , Hirsch Ian , Humphries Holly E. , Jepson Brett , Kelly Elizabeth J. , Plested Emma , Shoemaker Kathryn , Thomas Kelly M. , Vekemans Johan , Villafana Tonya L. , Lambe Teresa , Pollard Andrew J. , Voysey Merryn ,   the Oxford COVID Vaccine Trial Group, Adlou Syed , Allen Lauren , Angus Brian , Anslow Rachel , Asselin Marie-Claude , Baker Natalie , Baker Philip , Barlow Thomas , Beveridge Amy , Bewley Kevin R. , Brown Phillip , Brunt Emily , Buttigieg Karen R. , Camara Susana , Charlton Sue , Chiplin Emily , Cicconi Paola , Clutterbuck Elizabeth A. , Collins Andrea M. , Coombes Naomi S. , Clemens Sue Ann Costa , Davison Melanie , Demissie Tesfaye , Dinesh Tanya , Douglas Alexander D. , Duncan Christopher J. A. , Emary Katherine R. W. , Ewer Katie J. , Felle Sally , Ferreira Daniela M. , Finn Adam , Folegatti Pedro M. , Fothergill Ross , Fraser Sara , Garlant Harriet , Gatcombe Laura , Godwin Kerry J. , Goodman Anna L. , Green Christopher A. , Hallis Bassam , Hart Thomas C. , Heath Paul T. , Hill Helen , Hill Adrian V. S. , Jenkin Daniel , Kasanyinga Mwila , Kerridge Simon , Knight Chanice , Leung Stephanie , Libri Vincenzo , Lillie Patrick J. , Marinou Spyridoula , McGlashan Joanna , McGregor Alastair C. , McInroy Lorna , Minassian Angela M. , Mujadidi Yama F. , Penn Elizabeth J. , Petropoulos Christos J. , Pollock Katrina M. , Proud Pamela C. , Provstgaard-Morys Samuel , Rajapaska Durga , Ramasamy Maheshi N. , Sanders Katherine , Shaik Imam , Singh Nisha , Smith Andrew , Snape Matthew D. , Song Rinn , Shrestha Sonu , Sutherland Rebecca K. , Thomson Emma C. , Turner David P. J. , Webb-Bridges Alice , Wrin Terri , Williams Christopher J. 
Published: September 2021

The global supply of COVID-19 vaccines remains limited. An understanding of the immune response that is predictive of protection could facilitate rapid licensure of new vaccines. Data from a randomized efficacy trial of the ChAdOx1 nCoV-19 (AZD1222) vaccine in the United Kingdom was analyzed to determine the antibody levels associated with protection against SARS-CoV-2. Binding and neutralizing antibodies at 28 days after the second dose were measured in infected and noninfected vaccine recipients. Higher levels of all immune markers were correlated with a reduced risk of symptomatic infection. A vaccine efficacy of 80% against symptomatic infection with majority Alpha (B.1.1.7) variant of SARS-CoV-2 was achieved with 264 (95% CI: 108, 806) binding antibody units (BAU)/ml: and 506 (95% CI: 135, not computed (beyond data range) (NC)) BAU/ml for anti-spike and anti-RBD antibodies, and 26 (95% CI: NC, NC) international unit (IU)/ml and 247 (95% CI: 101, NC) normalized neutralization titers (NF50) for pseudovirus and live-virus neutralization, respectively. Immune markers were not correlated with asymptomatic infections at the 5% significance level. These data can be used to bridge to new populations using validated assays, and allow extrapolation of efficacy estimates to new COVID-19 vaccines.