Vaccine Efficacy of a Newly Developed Trivalent Vaccine Against Streptococcus iniae , Vibrio vulnificus and Vibrio harveyi in Asian Sea

ABSTRACT Vibriosis and Streptococcosis, caused by Vibrio and Streptococcus species, are major bacterial diseases affecting Asian seabass ( Lates calcarifer ) aquaculture in Vietnam. In the current study, V. harveyi , V. vulnificus and S. iniae were isolated with high relevance. A trivalent vaccine was developed using formalin‐inactivated representative strains ( V. harveyi IVRD 105, V. vulnificus IVRD 2.3, S. iniae IVRD B1) formulated with Montanide ISA 763 VG adjuvant and administered via intraperitoneal injection. Vaccinated fish exhibited significantly elevated serum antibody levels starting from 1 week post‐vaccination, peaking at 4 weeks. Challenge tests at 4 weeks post‐vaccination demonstrated high protective efficacy, with relative percent survival (RPS) values ranging from 76.09% to 89.63% against individual pathogens. Long‐term monitoring revealed sustained antibody titers and protection, with maximal survival rates of 91.50% against S. iniae , 80.07% against V. vulnificus and 83.77% against V. harveyi at 42 days post‐vaccination. These results demonstrate that the trivalent vaccine is immunogenic and effective in preventing Vibriosis and Streptococcosis in Asian seabass. This study provides the first report of a trivalent vaccine against these three pathogens in Vietnamese aquaculture, offering a promising strategy to reduce bacterial disease–related losses and improve farm sustainability.