Анотация: Objective--To investigate safety and efficacy of a cyprinid herpesvirus type 3 (CyHV3) modified-live virus vaccine for the prevention of koi herpesvirus disease (KHVd). Animals--420 healthy koi (Cyprinus carpio koi). Procedures-- Fish were vaccinated with a 1X dose or 10X overdose of CyHV3 modified-live virus vaccine or a placebo through bath exposure in tanks at 22рC. Horizontal transmission of vaccine virus was evaluated by commingling unvaccinated and vaccinated fish. Efficacy was evaluated by challenge exposure of vaccinated and naive fish to a wild-type virus. Fish that died were submitted for quantitative PCR assay for CyHV3 and histologic evaluation. Results--The CyHV3 vaccine was safe and efficacious, even at a 10X overdose. Vaccineassociated mortality rate was inversely associated w ith body weight, with a cumulative mortality rate of 9.4 (18/192) in fish weighing < 87 g and no deaths in fish weighing > 87 g (0/48). Horizontal transfer of vaccine virus from vaccinates to naive fish was negligible. For efficacy, the vaccine provided a significant reduction in mortality rate after challenge exposure to a wild-type virus, w ith a prevented fraction of 0.83 versus the placebo control fish. Conclusions and Clinical Relevance-- KHVd is highly contagious and commonly leads to deaths in 80 to 100 percent of exposed fish, representing a major threat to koi and common carp populations throughout the world. The CyHV3 modified-live virus vaccine had a favorable safety profile and was an effective vaccine for the control of KHVd in koi weighing > 87 g