Recombinant GP expressed from insect cells or a DNA vaccine based on GP only partially protected guinea pigs, but use of a combination of both vaccines resulted in 100% survival of guinea pigs [243] . In another study, complete protection of guinea pigs using a different GP DNA vaccine was reported, but only four of six vaccinated NHPs survived the challenge with MARV, showing incomplete protection [247] . Increased doses of a codon-optimized DNA vaccine resulted in 100% survival of NHPs, although some animals developed symptoms before recovering. In comparison to other vaccine candidates, a poor induction of virus-specific antibodies was observed using a DNA vaccine [248] . A codon-optimized DNA vaccine elicited a strong antibody response and resulted in complete protection of mice with no clinical symptoms observed [249] . Vaccine candidates based on the Venezuelan equine encephalitis virus (VEEV) replicon system expressing either MARV GP along with NP or GP alone completely protected guinea pigs and NHPs [250] .