Gumboro Disease Special
Figure 4. Protection expected when using a live Recombinant rHVT-VP2 vector type IBD vaccine. Protection
Active Immunity induced by: rHVT-VP2 vecto vaccine
Passive Immunity (MDA) Protection gap (VH)
Very high Virus Pressure
Protection gap (H)
High Virus Pressure
No protection gap (L)
Low Virus Pressure Age
FARM IBDV
conducted, etc. etc. are factors, among many others, that can explain the significant differences observed between vaccines that are provided with similar ‘rHVT-VP2’ generic names although with different commercial names. For this reason, it is important to state that the information given in this article regarding rHVT-VP2 vaccines and in particular conclusions regarding their properties and recommendations regarding their use relate to the rHVT-VP2 vaccines currently available on the market. These are the ones that we have analysed, tested and challenged. This information could prove to be inaccurate for other constructs based on similar rHVT-VP2 construction scheme or for any other recombinants, vector or not, that are expected to come in the future. The safety of live rHVT-VP2 vector IBD vaccines is identical to the safety of the HVT vaccine, that is to say excellent. No
negative effect has ever been reported in the field regarding this vaccine virus, widely used, and for almost 50 years. This safety is the really attractive feature of this category of vaccines.
Efficacy and dynamics of the immune response (see Figure 4) Contrary to live attenuated IBD vaccines where complete protection appears within a couple of days following replication of a whole virus, protection against Gumboro Disease induced by a rHVT-VP2 vector vaccine comes from the immune response of the chicken to the VP2 antigen expressed by rHVT-VP2 infected cells. It builds up progressively from a few days to several weeks after injection, so the level of protection depends very much on the age at which chickens are challenged. Experimentally, it is easy to show that the protection level against the challenge increases over time and needs
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