Randomized, placebo-controlled trial to assess the safety and immunogenicity of an adenovirus type 35-based circumsporozoite malaria vaccine in healthy adults

Abstract

Malaria results in over 650 000 deaths each year; thus, there is an urgent need for an effective vaccine. Pre-clinical studies and recently reported human trials suggest that pre-erythrocytic stage vaccines can provide protection against infection. A Phase 1, randomized, placebo-controlled, dose-escalation study was conducted with a vaccine composed of a replication-deficient adenovirus-35 backbone with P. falciparum circumsporozoite (CS) surface antigen (Ad35.CS.01). Healthy adult subjects received three doses of 108, 109, 1010, or 1011 vp/mL Ad35.CS.01 vaccine or saline placebo intramuscularly at 0, 1, and 6-mo intervals. Adverse events were assessed and anti-CS antibody responses were determined by ELISA. Seventy-two individuals were enrolled, with age, gender, and ethnicity similar across each study arm. While the vaccine was generally well tolerated, adverse events were more frequent in the highest dose groups (1010 and 1011 vp/mL). More robust humoral responses were also noted at the highest doses, with 73% developing a positive ELISA response after the three dose series of 1011 vp/mL. The Ad35.CS.01 vaccine was most immunogenic at the highest dosages (1010 and 1011 vp/mL). Reactogenicity findings were more common after the 1011 vp/mL dose, although most were mild or moderate in nature and resolved without therapy.