NIAID, IAVI, Emergent, and Scripps have developed two recombinant adenovirus type 4 (Ad4) vector-based vaccine candidates. These replicating Ad4 vector-based candidates have shown improved activity against tier 2 HIV-1 isolates in experimental animals. Tier 2 isolates are among the most prevalent in infected populations. The two candidates, Ad4-Env150KN and Ad4-Env145NFL, incorporate novel design features based on Ad4-EnvC150 (1086c). Specifically, the truncation of the cytoplasmic tail of Env increases cell surface expression and has resulted in improved antigenicity from both candidates.

Additionally, the upper respiratory tract administration offers a way to bypass pre-existing Ad4 immunity in most people. Furthermore, unlike non-replicating vectors, these vaccines may evoke a durable immune response.