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In farmed saltwater fish, vaccination is considered the first barrier against pathogenic agents. In Egypt, the vaccine application can diminish the dilemma of summer mass kills of farmed marine fish. The efficacy of booster vaccination with a bivalent vaccine against vibriosis was studied in European seabass. Fish were vaccinated by direct immersion (DI) in water containing bivalent Vibrio alginolyticus and Vibrio parahemolyticus whole cells killed vaccine. Immersion vaccination is more applicable under semi‐intensive fish culture, but less potent than injection. Accordingly, to increase antigen uptake and conferred protection, vaccinated fish randomly received booster vaccination via direct immersion (DI imm.) or intraperitoneal injection (i.p.) at 21 days post‐prime vaccination (21 dpv). Vaccinated and unvaccinated fish were monitored for 3 months and challenged with V. alginolyticus and/or V. parahemolyticus at 42 dpv. Specific and non‐specific immune responses were evaluated in blood samples. At 52 dpv (10 days post‐challenge), the relative immunoglobulin T (IgT) expression was assessed in the gills, hindgut, head kidney and spleen. Notably, imm. + imm. and imm. + i.p. vaccination yielded significantly higher specific antibody titre at 70 dpv, whereas the former was associated with significantly higher IgT expression in the gills and hindgut (p <.05) than the latter. High IgT levels in mucosal organs (gills and hindgut) might be the first indicator of the involvement of IgT in mucosal immune responses. Relative per cent survival among booster‐vaccinated fish ranged from 70% to 90%. The results support the use of this vaccination protocol as a highly effective tool to protect seabass from vibriosis and to improve fish health and environment. Booster doses are strongly recommended under farming environments. [ABSTRACT FROM AUTHOR]