17A replication-competent, vesicular stomatitis virus vaccine expressing the Ebola virus (EBOV) 18 glycoprotein (GP) (rVSV-ZEBOV) was successfully used during the 2013-16 EBOV epidemic 1 . 19 Additionally, chimeric and human monoclonal antibodies (mAb) against the EBOV GP showed 20 promise in animals and EBOV patients when administered therapeutically 2-6 . Given the large 21 number of at-risk humans being prophylactically vaccinated with rVSV-ZEBOV, there is 22 uncertainty regarding whether vaccination would preclude use of antibody treatments in the event 23 of a known exposure of a recent vaccinee. To model a worst-case scenario, we performed a study 24 using rhesus monkeys vaccinated or unvaccinated with the rVSV-ZEBOV vaccine. One day after 25 vaccination, animals were challenged with a uniformly lethal dose of EBOV. Five vaccinated 26 animals and five unvaccinated animals were then treated with the anti-EBOV GP mAb-based 27 therapeutic MIL77 starting 3 days postexposure. Additionally, five vaccinated macaques received 28 no therapeutic intervention. All five macaques that were vaccinated and subsequently treated with 29 MIL77 showed no evidence of clinical illness and survived challenge. In contrast, all five animals 30 that only received the rVSV-ZEBOV vaccine became ill and 2/5 survived; all five macaques that 31 only received MIL77 only also became ill and 4/5 survived. Enhanced efficacy of vaccinated 32 animals that were treated with MIL77 was associated with delayed EBOV viremia attributed to 33 the vaccine. These results suggest that rVSV-ZEBOV augments immunotherapy. 34 35 36 37 38 Outbreaks of filovirus disease have become increasingly difficult to manage due to 39 increased connectivity in endemic regions coupled with inadequate public health infrastructures 40 and lack of approved medical countermeasures including diagnostics, therapeutics, and vaccines. 41 Due to the sporadic nature of these outbreaks, the development and efficacy testing of preventative 42 vaccines and postexposure treatments has previously been limited to animal models, including 43 nonhuman primates, in which complete protection from lethal EBOV challenge has been 44 demonstrated 7-9 . The unprecedented magnitude of the 2013-16 West African EBOV epidemic 45 offered a unique opportunity to assess the efficacy of some of the most promising medical 46 countermeasures available at that time 7,8 . Notably, the rVSV-ZEBOV vaccine was shown to 47 provide 100% efficacy (95% CI 68·9-100·0, p=0·0045) when used in Guinea in a ring vaccination, 48 open-label, cluster-randomized Phase III clinical trial 1 . The same vaccine has reportedly shown 49 similar levels of success on a larger scale in the current outbreak of EBOV in the Democratic 50 Republic of Congo (DRC), having been administered to over 200,000 people 10 . Built on the 51 successes of years of development and validation in the field during two major ebolavirus 52 outbreaks, the rVSV-ZEBOV vaccine (licensed as Ervebo TM ) was recently approved for human 53 use by both the US...