Biosecurity measures are used in order to reduce the risk of introduction and spread of disease agents. In terrestrial animals vaccination is not included as a biosecurity measure. However, in fish farming where segregation, cleaning and disinfectionis difficult or impossible, vaccination is an important tool in order to prevent that pathogens establish and spread. Vaccination reduces the risk for disease outbreaks and serves as a firewall which prevents or slows down the spread of disease within farmed fish populations as well as to wild fish (Gudding et al, 2013)Herd immunity which means that unvaccinated fish is protected by a vaccinated majority is crucial in fish vaccinology where immunity in all individuals may be difficult to achieve. Vaccination reduces the basic reproduction number, R0, which is a parameter of the number of secondary cases arising from a single primary case in a susceptible population. A reduction of R to less than 1 means that an epizootic is less likely to develop or the epizootic declines.
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