During Phase 1/2 trials of the ChAdOx1 nCoV-19 vaccine, a small number of individuals exhibited significantly higher baseline levels of serum neutralizing antibodies compared to the rest of the cohort. This implies they were likely previous asymptomatic carriers of SARS-CoV-2 who had already developed a stable memory T-cell response prior to vaccination.

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Phase 1/2 clinical trials of the ChAdOx1 nCoV-19 (AZD1222) vaccine demonstrated an acceptable safety profile and robust immunogenicity. Mild systemic and local reactions, such as fatigue, fever, and headache, were common but peaked on the first day and were mitigated by prophylactic paracetamol. Immunologically, the vaccine induced spike-specific effector T-cell responses that peaked by week two, and serological antibodies that peaked by week four. A second dose further enhanced neutralizing antibody titers, although it did not significantly increase cellular immune responses.

ChAdOx1 nCoV-19 Phase 1/2 Trial Results

Folegatti, P. M., Ewer, K. J., Aley, P. K., Angus, B., Becker, S., & Belij-Rammerstorfer, S. (2020, July 20). Safety and immunogenicity of the ChAdOx1 nCoV-19 vaccine ... Retrieved August 17, 2020, from https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31604-4/fulltext

Safety and immunogenicity of the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2: a preliminary report of a phase 1/2, single-blind, randomised controlled trial

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