The purpose of this study is to assess the safety and tolerability of Modified Vaccinia Ankara (MVA) Mosaic vaccine in healthy adult participants.

This is a Phase I, placebo-controlled (the use of an inactive substance identical in appearance to the active vaccine), double-blind (neither the participant or study personnel will know the identity of the treatment administered) study where participants will be randomized (treatment type assigned by chance) to receive a Modified Vaccinia Ankara (MVA) Mosaic vaccine (at 1x10E8 pfu) or placebo. This design is intended to reduce the likelihood of observer and selection bias, provide control for confounding variables, and aid an unbiased analysis of the study results. The study will include 4 groups of participants, 2 groups having previously been vaccinated with Ad26.ENVA.01 (A recombinant adenovirus [rAd] vaccine for HIV-1) and 2 groups not previously vaccinated with Ad26.ENVA.01. Participants will be randomized in a 4:1 ratio to receive either a MVA Mosaic vaccine or placebo. The trial comprises a 4-week screening period, a 12-week vaccination period during which participants will be vaccinated at baseline (Day 1) and Week 12 (Day 84), and a 40-week follow-up period to the final visit at Week 52.

Inclusion Criteria:

• Healthy adults (determined by medical history, physical examination, and clinical judgment)
• HIV uninfected
• Female participants of child bearing potential must have a negative serum β-human chorionic gonadotrophin pregnancy test at the screening visit and immediately prior to each vaccine/placebo administration, practice adequate birth control measures from 28 days prior to the first vaccine/placebo administration through to at least 3 months after the final vaccine/placebo administration
• Male participants who are sexually active with a woman of childbearing potential and has not had a vasectomy must agree to use a double barrier method of birth control, e.g. either condom with spermicidal foam/gel/film/cream/suppository or partner with occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository

Exclusion Criteria:

• Confirmed HIV-1/-2 infection
• Chronic active hepatitis B or hepatitis C or active syphilis infection. Active syphilis documented by exam or serology unless positive serology is due to past treated infection
• Within the 12 months prior to enrollment: a history of newly acquired syphilis, gonorrhea, non-gonococcal urethritis, herpes simplex virus type 2 (HSV2), Chlamydia, pelvic inflammatory disease (PID), trichomonas, mucopurulent cervicitis, epididymitis, proctitis, lymphogranulomavenereum, chancroid, or hepatitis B
• A woman who is breastfeeding
• Any clinically significant acute or chronic medical condition that, in the opinion of the investigator, would preclude participation
• Major surgery within the 4 weeks prior to study entry or planned major surgery through the course of the study