Temperature during vaccination shapes antibody responses and protection against infectious salmon anemia in Atlantic salmon under different prime-boost vaccine strategies.

Vaccination is crucial for Atlantic salmon farming, protecting against bacterial and viral infections, such as infectious salmon anemia (ISA) caused by infectious salmon anemia virus (ISAV). Salmon immune responses are highly temperature-dependent and optimal water temperatures are expected to impact vaccine efficacy, while those out of the optimal range may weaken immunity and compromise protection. Additionally, vaccination regime impacts the length of protection against most common pathogens. To assess the impact of temperature and dosing regime on commercial vaccination, we evaluated two commercial multivalent vaccines in salmon reared at 8°C, 12°C, and 15°C. Fish were intraperitoneally injected with 100 μL of either vaccine 1 (V1) or vaccine 2 (V2). Half of each group received a booster after, between 700 and 750-degree days (dd), including groups that were boosted with the opposite vaccination (V1/V2; V2/V1). After an additional ∼650 dd post boost (∼1400 dd post first vaccination), ten fish from each group were sampled for serum IgM detection. One week later, donor fish previously infected with ISAv (ISAV-HPR4 at TCID50 of 1 × 105/ml) were introduced to cohabitation tanks at a 6.5:1 ratio of cohabitants to donors. Post-infection, survival rate was recorded, and serum samples were collected for specific-IgM detection, as well as head kidney to determine ISAv load. The results show that elevated rearing temperatures (12-15°C) consistently enhanced specific antibody responses against ISAv, A. salmonicida, and V. anguillarum, whereas cold conditions (8°C) limited or delayed antibody-mediated immune responses in pre-exposed fish. Booster vaccinations and higher temperatures effectively increased and maintained IgM levels in pre-exposed Atlantic salmon, compensating for low-temperature suppression. Survival and viral load data further highlight the interaction between temperature and immune protection, with fish hold at warmer temperatures during vaccination exhibiting higher survival and more efficient ISAv clearance. These findings demonstrate that water temperature and vaccination strategy, including regimen and formulation, critically influence adaptive immunity in Atlantic salmon. Aligning vaccination protocols with seasonal and environmental conditions can maximize protection and limit pathogen persistence.