Interim Results of a Phase 1-2a Trial of Ad26.COV2.S Covid-19 Vaccine

Abstract

Background: Efficacious vaccines are urgently needed to contain the ongoing coronavirus disease 2019 (Covid-19) pandemic of infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A candidate vaccine, Ad26.COV2.S, is a recombinant, replication-incompetent adenovirus serotype 26 (Ad26) vector encoding a full-length and stabilized SARS-CoV-2 spike protein.

Methods: In this multicenter, placebo-controlled, phase 1-2a trial, we randomly assigned healthy adults between the ages of 18 and 55 years (cohort 1) and those 65 years of age or older (cohort 3) to receive the Ad26.COV2.S vaccine at a dose of 5×10¹⁰ viral particles (low dose) or 1×10¹¹ viral particles (high dose) per milliliter or placebo in a single-dose or two-dose schedule. Longer-term data comparing a single-dose regimen with a two-dose regimen are being collected in cohort 2; those results are not reported here. The primary end points were the safety and reactogenicity of each dose schedule.

Results: After the administration of the first vaccine dose in 805 participants in cohorts 1 and 3 and after the second dose in cohort 1, the most frequent solicited adverse events were fatigue, headache, myalgia, and injection-site pain. The most frequent systemic adverse event was fever. Systemic adverse events were less common in cohort 3 than in cohort 1 and in those who received the low vaccine dose than in those who received the high dose. Reactogenicity was lower after the second dose. Neutralizing-antibody titers against wild-type virus were detected in 90% or more of all participants on day 29 after the first vaccine dose (geometric mean titer [GMT], 212 to 354), regardless of vaccine dose or age group, and reached 96% by day 57 with a further increase in titers (GMT, 288 to 488) in cohort 1a. Titers remained stable until at least day 71. A second dose provided an increase in the titer by a factor of 2.6 to 2.9 (GMT, 827 to 1266). Spike-binding antibody responses were similar to neutralizing-antibody responses. On day 15, CD4+ T-cell responses were detected in 76 to 83% of the participants in cohort 1 and in 60 to 67% of those in cohort 3, with a clear skewing toward type 1 helper T cells. CD8+ T-cell responses were robust overall but lower in cohort 3.

Conclusions: The safety and immunogenicity profiles of Ad26.COV2.S support further development of this vaccine candidate. (Funded by Johnson & Johnson and the Biomedical Advanced Research and Development Authority of the Department of Health and Human Services; COV1001 ClinicalTrials.gov number, NCT04436276.).

Figure 1. Solicited Adverse Events in Cohorts 1 and 3 after the First Vaccine Dose.

Shown are solicited adverse events in participants who received the Ad26.COV2.S vaccine at a dose of 5×10¹⁰ viral particles (low dose) or 1×10¹¹ viral particles (high dose) per milliliter or placebo. Healthy adults between the ages of 18 and 55 years were included in cohort 1 (Panel A), and those 65 years of age or older were included in cohort 3 (Panel B). The younger group was divided into cohorts 1a and 1b, with the latter designated as an exploratory cohort for in-depth analysis of immunogenicity. As shown here, data for cohorts 1a and 1b have been pooled. Data for patients in cohort 1a who received a second dose of vaccine are provided in Figure S2 in the Supplementary Appendix.

Figure 2. Humoral Immunogenicity.

Shown are measures of humoral immunogenicity in serum samples obtained from the participants in cohort 1a (left side) and cohort 3 (right side), according to the receipt of the low or high dose of Ad26.COV2.S or placebo. In cohort 1a, the participants received two injections of high-dose or low-dose vaccine or placebo, as indicated with slashes (e.g., placebo/placebo if they received two injections of placebo). The samples were measured on enzyme-linked immunosorbent assay (ELISA) in ELISA units (EU) per milliliter (Panel A) and on wild-type virus neutralization assay, with seropositivity defined as a half maximal inhibitory concentration (IC₅₀) titer of more than 58 at the lower limit of quantitation (Panel B). Logarithmic values are reported as the geometric mean concentration (GMC) in the ELISA analyses and as the geometric mean titer (GMT) in the neutralizing-antibody analyses. The values were measured at baseline and at day 29 after vaccination in all the participants and on days 57 and 71 in those in cohort 1a. The two horizontal dotted lines in each panel indicate the lower and upper limits of quantitation of the respective assay; values below the lower line have been imputed to half the lower limit of quantitation. 𝙸 bars indicate 95% confidence intervals. HCS denotes human convalescent serum.

Figure 3. Cellular Immunogenicity of Ad26.COV2.S.

In CD4+ T cells, the response to low-dose or high-dose vaccine or placebo in type 1 helper T (Th1) cells was characterized by the expression of interferon-γ, interleukin-2, or both, without cytokines expressed by type 2 helper T (Th2) cells (Panel A). The response in CD4+ Th2 cells was characterized by the expression of interleukin-4, interleukin-5, or interleukin-13 (or all three cytokines) plus CD40L (Panel B). In CD8+ T cells, the response was measured by the expression of interferon-γ, interleukin-2, or both (Panel C). In all three panels, the horizontal bars indicate median values on intracellular cytokine staining for individual responses to a SARS-CoV-2 S protein peptide pool in peripheral-blood mononuclear cells at baseline and 15 days after vaccination in a subgroup of participants in cohort 1a (left side) and cohort 3 (right side), according to the receipt of the low or high dose of Ad26.COV2.S or placebo. The horizontal dotted line in each panel indicates the lower limit of quantitation (LLOQ); values below the line have been imputed to half the LLOQ.