Longevity of protection in cattle following immunisation with emergency FMD A22 serotype vaccine from the UK strategic reserve
To determine the longevity of protective immunity following a single administration of emergency vaccine, and establish whether the immune response could be enhanced by increasing the antigen payload even further, cattle were vaccinated with an A22 Iraq vaccine containing either 1 x antigen payload (field dose) or 5x antigen payload. Six months post-immunisation all cattle received a homologous virus challenge. The magnitude of the virus neutralising antibody response elicited was consistent with the response to similarly formulated A serotype vaccines with a PD(50) greater than 32. All the vaccinated cattle, regardless of antigen payload, were protected from clinical disease following challenge although some cattle in both groups became sub-clinically infected. We conclude that immunisation with a single inoculation of vaccine from the UK emergency reserve can protect cattle from clinical disease for at least 6 months post-vaccination and that a boost may be unnecessary in an outbreak situation. Some animals may become sub-clinically infected but this is likely to be dependent on the severity of challenge. The study confirmed that a booster at 21 days post-vaccination was not necessary to maintain a cell-mediated response in cattle for 6 months. No increased benefits were recognised by increasing the antigen payload of this vaccine 5x.