The differences in the survival rates of the formalin-treated
vaccine groups in trial I and trial II were attributed to the difference in the formalin-treatment period:
the formalin-treated VHSV administered in trial I was not completely inactivated and worked as a live
vaccine, which explains the 80% survival rate against VHSV challenge. Specific antibodies against VHSV
were detected in sera from all vaccinated survivors, except the DEPC-VHSV 4 group. Furthermore, the
specific antibody titers of fish vaccinated with the live and dead VHSV vaccines were similar, but the
protective effects of the live and dead vaccines varied considerably. Our findings show that Poly(I:C)
immunization with the live vaccine offers better protection than the formalin-treated vaccine against
VHS in olive flounder and revealed that antibody levels are not a reliable indicator of the protective effect
of the vaccine against the pathogen. In the future, elements of T cell immunity may be used as a means of
evaluating the protective efficacy of a vaccine against VHSV instead of ELISA.