Commercial vaccines do not confer protection against two genetic strains of Piscirickettsia salmonis, LF-89-like and EM-90-like, in Atlantic salmon

In Atlantic salmon, vaccines have failed to control and prevent Piscirickettsiosis, for reasons that remain elusive. In this study, we report the efficacy of a commercial vaccine developed with the Piscirickettsia salmonis isolate AL100005 against other two isolates which are considered highly and ubiquitously prevalent in Chile: LF-89-like and EM-90-like. Two cohabitation trials were performed to mimic real-life conditions and vaccine performance: 1) post smolt fish were challenged with a single infection of LF-89-like, 2) adults were coinfected with EM-90-like and a low coinfection of sea lice. In the first trial, the vaccine delayed smolt mortalities by two days; however, unvaccinated and vaccinated fish did not show significant differences in survival (unvaccinated: 60.3%, vaccinated: 56.7%; p = 0.28). In the second trial, mortality started three days later for vaccinated fish than unvaccinated fish. However, unvaccinated and vaccinated fish did not show significant differences in survival (unvaccinated: 64.6%, vaccinated: 60.2%, p= 0.58). Thus, we found no evidence that the evaluated vaccines confer effective protection against of LF-89-like or EM-90-like with estimated relative survival proportions (RPSs) of −9% and −12%, respectively. More studies are necessary to evaluate whether pathogen heterogeneity is a key determinant of the vaccine efficacy against P. salmonis. ### Competing Interest Statement We declare that J.A.G., L.M., and P.C. provided genetic and immunological services to different salmon companies in Chile during the execution of this experiment. G.S and C.S. were employed in Salmones Camanchaca during the execution of this research. C.F., D.T., B.M-L, and B.D. declare no competing financial interest.