A Formalin-Inactivated Vaccine Enhances Survival and Mitigates Horizontal Transmission of Red Sea Bream Iridovirus (RSIV) in Rock Bream (Oplegnathus fasciatus): Insights From Viability Quantitative PCR.

Formalin-inactivated vaccines are widely employed as a primary preventive strategy against red sea bream iridoviral disease (RSIVD), which poses a substantial economic threat to the aquaculture of rock bream (Oplegnathus fasciatus). However, conventional quantitative PCR (qPCR) cannot differentiate infectious virions from noninfectious vaccine residues or viral debris, limiting accurate assessment of vaccine efficacy. Therefore, the present study aimed to evaluate the protective efficacy and viral shedding dynamics of a formalin-inactivated vaccine using propidium monoazide (PMAxx)-based viability qPCR (vqPCR). Vaccination demonstrated robust protection, achieving a relative percent survival of approximately 80% in immersion challenges and significantly reducing viral shedding into seawater. Notably, comparative analysis revealed that conventional qPCR significantly overestimated viral risk in vaccinated fish by detecting noninfectious DNA artifacts. Although vaccination did not confer sterilizing immunity, it suppressed infectious viral replication to sublethal levels, effectively preventing horizontal transmission to naïve cohabitants. Furthermore, disease progression and shedding kinetics were temperature-dependent, occurring more rapidly at 25°C than at 20°C. Overall, these findings highlight that vaccination induces functional sterilization of shedding and underscore the need to adopt vqPCR for accurate epidemiological risk assessment in aquaculture populations.