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With FMD vaccines, the word ‘Virus’ is usually used to denote an infectious preparation etc. whereas the word ‘Antigen’ is used to denote an inactivated preparation.
FMD exists as seven serotypes and very many antigenically distinct strains within the serotypes. The A serotype is particularly heterogenous and, for practical purposes, the A22 lineage of viruses is treated as a separate serotype from the A24 lineage.
A working definition of serotype is that immunity to one serotype does not confer protection against any of the other serotypes and, in practice, dominant strains within each serotype. With the O serotype, for example, O Manisa is widely used in vaccines for the Middle East and South East Asia whereas O Campos is the dominant O strain for use in South America.
The properties of the seven serotypes are so diverse that it is sometimes useful to think of them as seven different viruses.
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