The first killed vaccine was developed to prevent trout losses caused by S. iniae infection [28]. Killed vaccines are generally administered through injection to achieve high efficacy. Previous research demonstrated that bath immunization of tilapia resulted in RPS values that were 2-fold lower than those achieved using i.p. vaccination [29]. In a field trial, Eldar et al. [28] reported that the mortality of rainbow trout intraperitoneally immunized with a formalin-killed S. iniae vaccine was 5%, but exceeded 50% in non-immunized rainbow trout. Similarly, the results of this study demonstrated that i.p. injection of a formalin-killed S. iniae vaccine provides a higher level of protection in groupers (RPS, 97.5%–100%) after S. iniae challenge; such a vaccine can provide excellent efficacy against S. iniae infection in fish.
Oil-based vaccines administered to fish provide long-term pro- tection [30]. In this study, we posit that whether antigens are mixed with the ISA763A or AS-F adjuvant, adequate protection is achieved in the vaccinated groupers. Protection conferred by the tested vac- cine lasted a minimum of 6 months, as determined in the laboratory challenge. As streptococcosis poses a threat to groupers during the