Vectored Vaccines: a Step Forward for Disease Control and Surveillance

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Abstract

Control and eradication of avian influenza outbreaks in the pre-H5N1 Endemic Era almost uniquely relied upon the use of traditional inactivated vaccines, targeting high-risk populations in selected areas. The rare occurrence of HPAI epidemics was mainly brought under control with the adoption of stamping-out policies and movement control, within a year from the detection of the case index. When Asia faced the unprecedented event of multiple outbreaks of HPAI H5N1 in 2003-2004, mass culling soon appeared to be ineffective and economically unsustainable, leading veterinary authorities to the implementation of massive vaccination campaigns. From a wider perspective, the devastating H5N1 epizootic indeed acted as a potent booster for the development of new vaccine technologies such as reverse genetics engineered viruses, viruslike particles, subunit vaccines, DNA vaccines and viral vectored vaccines.

Vectored vaccines are currently considered the most promising platform for the delivery of immunogens capable of inducing a broad, long-lasting resistance to infection, involving both humoral and protective cellular immunity.

In a study conducted at IZSVe, Italy, a newly approved vector vaccine consisting of a herpesvirus of turkey expressing the haemagglutinin gene of the HPAI H5N1 clade 2.2 A/Swan/Hungary/499/2006 virus (rHVT-H5) was tested to investigate its protective efficacy against a more recent, and genetically distant clade 2.3.2.1 HPAI H5N1 virus isolated in Bangladesh, in 2011.

Day-old SPF chicks were vaccinated with the rHVT-H5 vaccine, or kept as unvaccinated controls, and challenged at 28 days of age with a dose of 106 EID50.

Response to vaccination and protection from infection were evaluated on the basis of clinical observation, reduction of viral shedding, detection and quantisation of antibodies against both vaccine and virus antigens.

The vaccine conferred complete clinical protection and suppressed shedding of viable virus in 90 % of the challenged animals. The rHVT-H5 vaccine induced a robust immune response preventing infection in the majority of the chickens. Two birds shedding viral RNA were also positive by NP-ELISA, confirming detection of nucleoprotein antibodies as a valid tool for differentiating infected from vaccinated animals, provided that infection with other subtypes (e.g., H9N2) is excluded.

Bangladesh is one of the six countries where the HPAI H5N1 virus is still officially endemic and vaccination has not been implemented so far. The rHVT-H5 tested in this study may hence represent a powerful instrument for an alternative control strategy of the current epidemic.

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