ABSTRACT
Background: Ad26.COV2.S is a single-shot vaccine that has demonstrated clinical efficacy against symptomatic COVID-19. In this study, we report the durability of immune responses in 20 rhesus macaques received single-shot Ad26.COV2.S and the immunogenicity of a booster shot at 8-10 months following the initial immunization. Methods: Animals were immunized by intramuscular route with 1011 vp (N=10) or 5x1010 vp (N=10) Ad26.COV2.S and were followed for either 230 or 315 days. Animals were then boosted with 5x1010 vp Ad26.COV2.S (N=10). Humoral immune responses including RBD-specific Ig ELISA and pseudovirus-based virus neutralization response were monitored. Circulating RBD-specific memory B cells and bone marrow plasma cells were assessed by multiparameter flow cytometry. Results: Ad26.COV2.S elicited robust and comparable RBD-specific binding and neutralizing antibody responses in animals that received the 1011 vp and 5x1010 vp doses, which peaked on days 28-56, and then showed a biphasic decay. All animals showed binding antibody responses for the duration of follow-up, and 17 of 20 animals showed neutralizing antibody responses by day 230-315. RBD-specific memory B cell response peaked on day 14-28 followed by a gradual decline, and remained detectable in 17 of 20 animals by day 230-315. On day 315 following vaccination, bone marrow RBD-specific PCs were detected in the majority of vaccinated macaques, including in all animals that received the 1011 vp dose. Following Ad26.COV2.S boost immunization, RBD-specific binding antibody responses increased 31-69 fold compared with pre-boost levels against the ancestral (WA1/2020), alpha (B.1.1.7), beta (B.1.351), kappa (B.1.617.1), and delta (B.1.617.2) SARS-CoV-2 variants. Neutralizing antibody responses increased 23-43 fold compared with pre-boost levels against the ancestral, alpha, beta, gamma (P.1), kappa, and delta SARS-CoV-2 variants. Antigen-specific memory B cell response also increased 8 fold following the boost immunization. Conclusion: Ad26.COV2.S elicited durable antibody and B cell responses, and a late boost with Ad26.COV2.S resulted in a dramatic increase in humoral immunity that were highly cross-reactive across multiple SARS-CoV-2 variants in rhesus macaques. These data contribute to our understanding of Ad26.COV2.S durability and boostability, and provide important data to inform COVID-19 vaccine boosting strategies in humans.