BackgroundApproaches to achieving antiretroviral therapy (ART)-free remission from HIV-1 must consider that people over 50 years now comprise the majority of people with HIV (PWH) on ART in various regions, including the U.S.MethodsWe report a double-blind, randomized trial in which PWH on ART, aged 21-60 years, received modified vaccinia Ankara (MVA)-vectored vaccines, MVA.tHIVconsv3 (M3) and MVA.tHIVconsv4 (M4), either alone or in combination (n=7/group) or saline placebo (n=3). M3 and M4 contain complementary HIVconsvX immunogens that each span the same regions in HIV-1 Gag and Pol but differ at approximately 8% at the amino acid level.ResultsM3, M4, and M3M4 regimens were well tolerated and all significantly increased both the frequency (peak median increase ~3-fold) and breadth of the HIVconsvX-specific T-cell response while redirecting T cells to target conserved regions in HIV-1 for up to 10 weeks post-vaccination. We also demonstrated that vaccination increased frequencies of T-cells targeting participant autologous HIV-1 sequences. Vaccination mostly expanded pre-existing HIV-1-specific T cells, did not impact CD4 T-cell activation, low-level viremia, or integrated HIV-1 provirus. Linear regression indicated that age was independently and negatively associated with the change in T-cell frequency at 1-, 2- and 10-weeks after vaccination (~1.41-fold decrease per 10 years older). After adjusting for age, years on ART was positively associated with HIVconsvX-specific T-cell frequencies at 1- and 2-weeks following vaccination.ConclusionIn PWH receiving ART, MVA.HIVconsvX vaccines significantly increased T cells targeting conserved regions of HIV-1. Novel strategies may be required to enhance anti-HIV-1 immunity in older adults.Trial registrationNCT03844386.