Safety and Humoral Immunogenicity of Different Dose Levels of Ad26.COV2.S as a 2-Dose Regimen in COVID-19 Vaccine-Naïve Healthy Adults: A Phase 3 Randomized Clinical Trial

Background: This study aimed to support the end-of-shelf life specification (2.5 × 10¹⁰ virus particles [vp]) for the standard Ad26.COV2.S dose (5 × 10¹⁰ vp).

Methods: This randomized, double-blind Phase 3 study evaluated immunogenicity, reactogenicity, and safety of several Ad26.COV2.S dose levels (range 1.25 to 9 × 10¹⁰ vp) in 1593 adults between June 2021 and July 2023.

Results: Spike-binding antibody responses 28 days post-dose 1 were non-inferior for the 9 × 10¹⁰ vp, but not the 2.5 × 10¹⁰ vp group when compared with the standard dose. Non-inferiority was demonstrated in terms of spike-binding antibody responses 14 days post-dose 2 for each dose level, including the lowest dose level of 1.25 × 10¹⁰ vp, compared to 28 days after one dose and 14 days after two doses of the standard dose. Spike-binding antibody levels correlated well with virus neutralizing titers. There was no impact of pre-existing Ad26.COV2.S neutralizing titers on immunogenicity at any dose level. All dose levels were well tolerated.

Conclusions: This study highlights the challenges associated with conducting clinical studies in a rapidly evolving environment and underscores the importance of platform data that can guide initial vaccine specifications such as shelf life during accelerated vaccine development. The present study supports the end-of-shelf life specifications for the approved Ad26.COV2.S dose, and could provide useful information in future vaccine developments using adenovirus vector vaccines.