WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2013 63 Spread of infection in the population can occur when there is contact between susceptible and infectious individuals and that contact results in successful transmission of the infection. In an attempt to minimize the number of new infections, vaccination has been used as a tool to reduce the contact that leads to disease transmission between infectious and susceptible individuals by converting susceptible to immune individuals. As a result, vaccination reduces the number of susceptible individuals that could have been infected after contact with infectious individuals. If the R0 of a disease and the efficacy of a particular vaccine are known, then the herd immunity for a population can be calculated (Table 1). Modeling of Vaccination In cultured fish populations, the consequences of vaccination can be described under different scenarios. Assume that a disease with R0 of 3 has entered a fish population. A mathematical model can be used to monitor the change in the numbers of susceptible, newly infected, and dead fish during the period of the epidemic. When the whole population is unvaccinated, all fish are susceptible to the infection. Once an infectious fish has entered the population, all susceptible fish are either infected or die within 90 days. The majority of fish die within 150 days, although the infection persists beyond that period (Fig. 1). Figure 2 illustrates a scenario where fish in a population are vaccinated with a vaccine having a RPS of 70 percent. Different proportions (40 percent, 80 percent, 90 percent, and 95 percent) of susceptible fish are vaccinated with this vaccine. Because of the relatively low RPS, the epidemic still develops even if 95 percent of susceptible fish are vaccinated. However, the greater the proportion of fish vaccinated, the smaller the magnitude of the epidemic. Even if a vaccine with a RPS of 80 percent is used, the epidemic will still proceed even if 95 percent of the fish are vaccinated. However, the magnitude of the epidemic would be much smaller compared to a vaccine with a RPS of 70 percent. Finally, 40, 80, 90 and 95 percent of susceptible fish in a population are vaccinated with a vaccine of 90 percent RPS (Fig. 3). To achieve complete protection of a disease epidemic with R0 of 3, at least 95 percent of the susceptible population would have to be vaccinated with a vaccine having a RPS of at least 90 percent. Supporting the herd immunity concept, the 5 percent of non-vaccinated or non-responding individuals have the benefit of disease prevention from this large proportion of vaccinated individuals. With an R0 less than 3, preventing an epidemic will occur at a lower proportion of vaccinated fish or after vaccinating with a less efficacious vaccine. Conclusions Although vaccination cannot, in general, be considered to be a preventive measure that completely eliminates the risk of infection of a population with a specific pathogen, the protection achieved may significantly reduce the number of susceptible individuals. The resulting herd immunity may effectively prevent an infection from becoming established and spread within a population and to other aquaculture installations. Notes 1 Norwegian Veterinary Institute, PO Box 750, Sentrum N-0106 Oslo, Norway roar.gudding@vetinst.no 2 Norwegian Veterinary Institute, PO Box 750, Sentrum N-0106 Oslo, Norway atle.lillehaug@vetinst.no *Corresponding author 3 Norwegian Veterinary Institute, PO Box 750 Sentrum N-0106 Oslo, Norway saraya.tavornpanich@vetinst.no References Amend, D.F. 1981. Potency testing of fish vaccines. Developments in Biological Standardization 49:447-454. Anderson, R. M. and R. M. May. 1991, Infectious Diseases of Humans: Dynamics and Control. 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