Salmonid Rickettsial Septicemia (SRS), caused by the bacterium Piscirickettsia salmonis, is the main reason for antibiotic usage in the Chilean aquaculture industry. In 2016, a live attenuated vaccine (ALPHA JECT LiVac^® SRS, PHARMAQ AS) was licensed in Chile and has been widely used in farmed salmonids since then. In experimental injection and cohabitation laboratory challenge models, we found that the vaccine is effective in protecting Atlantic salmon (Salmo salar) for at least 15 months against P. salmonis-induced mortality. However, the protection offered by the vaccine is sensitive to temperature during immunization. Fish vaccinated and immunized at 10 °C and above were well protected, but those immunized at 7 °C and 8 °C (the lower end of the temperature range commonly found in Chile) experienced a significant loss of protection. This temperature-dependent loss of effect correlated with the amount of vaccine-strain RNA detected in the liver the first week after vaccination and with in vitro growth curves, which failed to detect any growth at 8 °C. We found that good vaccine efficacy can be restored by exposing fish to 15 °C for the first five days after vaccination before lowering the temperature to 7 °C for the remaining immunization period. This suggests that maintaining the correct temperature during the first few days after vaccination is crucial for achieving a protective immune response with ALPHA JECT LiVac^® SRS. Our results emphasize the importance of temperature control when vaccinating poikilothermic animals with live vaccines.

Background/Objectives: This article reports results from a multisite field trial conducted as part of the regulatory evaluation of a novel monovalent vaccine for Atlantic salmon targeting an emerging variant of Moritella viscosa, with the objective of assessing safety and performance under commercial production conditions. Methods: The trial included approximately 8 million Atlantic salmon reared at seven freshwater and eleven seawater sites in Norway. At each site, fish in the test and control groups received identical vaccination regimens, with the test group additionally receiving the test vaccine. Fish were monitored from vaccination until harvest. Safety endpoints included post-vaccination mortality and local reactions; effectiveness endpoints included outbreak-related mortality and antibody responses. Harvest quality was evaluated as an exploratory endpoint. Results: Post-vaccination mortality within 21 days was low and comparable between groups. Local reaction scores were within acceptable ranges but slightly higher for the test group. Outbreaks of winter ulcer disease caused by variant M. viscosa occurred at three seawater sites, during which test groups were associated with substantially lower mortality compared with controls. At these sites, the proportion of downgraded fish at slaughter was consistently lower in test than control groups. Across sampling points, the test group showed higher antibody titers than the control group. Conclusions: Co-administration of the monovalent variant M. viscosa vaccine with commercial core vaccines was associated with reduced disease burden during outbreaks and a favourable safety profile under commercial farming conditions. These findings support its potential relevance in vaccination programmes for Atlantic salmon farming.