Summary Due to the persistent reservoir established soon after HIV-1 transmission, life-long ART is required to maintain viral load suppression in HIV-1 infected individuals. Development of strategies able to induce long-term, ART-free remission is a high public health priority. One important strategy is therapeutic HIV vaccine. Although to date, majority of therapeutic vaccines failed to show clinical benefit, some showed evidence of vaccine efficacy as a delayed time to rebound or decreased viral load set point. This provides the hope to achieve a long-term remission if optimized vaccine strategies can be developed. The proposed study leverages a highly unique opportunity to employ novel sequencing technologies to understand viral evolutionary patterns in response to therapeutic vaccination in a promising Ad26.Mos/MVA.Mos vaccine regimen amongst acutely treated individuals who underwent analytical treatment interruption (ATI) in the RV405 study. This will be the first HIV-1 genetic study in the setting of a randomized, placebo- controlled therapeutic vaccine trial. Preliminary analysis showed successfully elicited immune responses in all RV405 vaccine recipients and a slightly delayed time to viral load rebound in the vaccine arm. Our central hypothesis is that the immune responses elicited by Ad26/MVA vaccination will exert selection pressures on the replicating viruses during rebound, which will leave “genetic imprints” in the rebound viral genomes. Both the genetic background of the T/F virus and its evolutionary trajectory in response to vaccine-induced immune pressures will correlate with the vaccine outcomes. We propose the following specific aims: Aim 1: To compare the genetic compositions of pre-ART and rebound viral populations and to identify vaccine-related genetic imprints in rebound viral genomes. Aim 2: To identify viral genetic correlates of the vaccine outcomes in both the T/F and rebound viral genomes. Our long-term goal is to translate knowledge of HIV vulnerabilities into optimizing future vaccine strategies towards the goal of HIV remission and eradication.