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The chimeric flavivirus LGT/DEN4 containing prM and E genes of naturally attenuated Langat virus with remaining sequence derived from low neuroinvasive Dengue 4 virus was previously produced and assessed as a candidate for live vaccine against tick-borne encephalitis (TBE) [Pletnev and Men (1998): Proc Natl Acad Sci USA 95:1746-1751; Pletnev et al. (2000): Virology 274:23-31; Pletnev et al. (2001): J Virol 75:8259-8267; Wright et al. (2008): Vaccine 26:882-890]. In this article we compared two animal species: mice and monkeys, in order to select most sensitive models for safety evaluation of new vaccine candidates against TBE. Direct neurovirulence in suckling mice, neuroinvasiveness upon peripheral inoculation, rate of virus multiplication and expansion in CNS and its ability to persist in the central nervous system (CNS) were studied in adult mice; virological and pathomorphological examination of the CNS and visceral organs after intrathalamic virus inoculation was selected as a safety neurovirulence test in monkeys. The chimera was substantially less virulent in both animal models compared to the Absettarov strain of TBE virus. LGT/DEN4 was highly attenuated in suckling and adult mice with no evidence of viral persistence in CNS. In contrast to the mouse model, the chimera was able to reproduce in the CNS of monkeys to moderate titers, caused pathomorphological lesions in two and even illness in one of four animals, and was registered in simian brain on the 30th day post-infection. The presented data show that tests in mice solely might not be a sufficient model for safety testing of chimeric viruses.