Vampire bats represent major rabies virus reservoirs from Mexico to Argentina. To better understand host-virus infection and immunity associated with this hematophagous species, epidemiological parameters were compared retrospectively in historical outbreaks of paralytic rabies in livestock (BovOut) and humans (HumOut) without a prior vaccination history in two South American locales. Outbreaks occurred between 1968 and 1969 in San Borgita, Argentina, and in Santa Luzía, Brazil, between 1990 and 1991. In both outbreaks, some individuals became ill, dying with compatible signs of rabies. Other exposed individuals survived without becoming apparently ill, with or without the detection of rabies virus-neutralizing antibodies (RabVNA). There were no individuals who recovered spontaneously after showing central nervous system involvement. Observed mortality was significantly lower than survival. The prevalence of exposed survivors without detectable RabVNA was higher than that of survivors with RabVNA. There were no statistically significant differences between BovOut and HumOut in the prevalence of vampire bites, mortality, and survival; survivors without RabVNA and survivors with RabVNA. Such results are consistent with observations of rabies in vampires and other bat species and contrast with outbreaks in carnivores, in which mortality tends to be higher and survivors with RabVNA scarce. The overall within-host pathogenicity of RabV transmitted by bats may be lower than variants transmitted by carnivores, such as dogs. A lower level of relative virulence may have originated via co-evolution between RabV and bat populations. Such retrospective studies are useful for consideration in future rabies outbreaks, as related to surveillance parameters and disease management.