Correlates of severe and delta COVID-19 in a phase 3 trial of the AZD1222 vaccine.

Publication date: Apr 18, 2026

In the phase 3 AZD1222 COVID-19 vaccine trial, anti-Spike (vaccine-matched and Delta) binding IgG antibody concentration and neutralizing antibody (nAb) titer (vaccine-matched+D614G and Delta), measured four weeks post-dose two (D57), were assessed as correlates of risk of severe COVID-19 and Delta COVID-19 over ~4 to ~13 months (severe) or ~11 months (Delta) post-D57. Using a case-control design, antibodies were measured in baseline SARS-CoV-2-negative per-protocol ChAdOx1 nCoV-19 recipients (19 severe COVID-19 cases, 57 Delta COVID-19 cases, 111 controls). The hazard ratio (HR) of severe COVID-19 per 10-fold vaccine-matched D57 marker increase was 0. 16 (95% CI: 0. 05, 0. 54; p = 0. 004) for Spike IgG and 0. 13 (0. 03, 0. 59; p = 0. 009) for nAb titer. D57 Delta antibodies were weak correlates of Delta COVID-19: HR per 10-fold increase 0. 70 (0. 14, 3. 47; p = 0. 66) for Delta Spike IgG; 0. 46 (0. 14, 1. 47; p = 0. 19) for Delta nAb titer. Binding and nAb levels strongly predicted severe COVID-19, even with antibody waning.

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Semantics

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