Safety and Field Performance of a Monovalent Vaccine Against Variant Moritella viscosa in Atlantic Salmon Under Commercial Conditions

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.