Efficacy of a Vector Avian Influenza Vaccine Against Challenge
To assess protection of the birds against virulent Highly Pathogenic Avian Influenza virus (HPAIV) challenge, 2 criteria are important:
- Protection against mortality
- Reduction of shedding of the challenge virus after inoculation
In the experiments listed in the table below, the vector rHVT-HA5 vaccine was always injected subcutaneously on the first day of age and challenge given between 2 to 8 weeks of age with a dose of 105 or 106 EID50 of challenging HPAIV
Protection against mortality was generally good to very good, varying from 70% to 100% depending on the experiments. These results included challenges with strains of different clades of H5N1 (1, 2.2, 2.2.1, 2.2.1.1, 2.1.3, 2.3.2.1), or different subtypes (Mexico 1994 H5N2, Germany 2014 H5N8 clade 2.3.4.4) showing very significant antigenic differences from the strain used to construct the vaccine. This led to the conclusion that the vaccine offered “cross clades protection against HPAIV of the H5 type” ( #12 ).
(click on the below table reference to access trial details)
It is believed that immunity induced by this vaccine is of humoral (antibody) as well as of cellular types, and of general as well as of mucosal types ( #12 , #20 ). Obviously, the criteria generally used indirectly to assess protection induced by classical killed vaccines, and in particular antibody response to the challenging virus, are not applicable to this vaccine.
A significant reduction of shedding was observed, resulting from a lower percentage of shedders as well as less virus shed in all cases, to different degrees. It is believed that differences in experimental conditions and procedures (RT-PCRs of different types, virus isolation) help explain these variations.
A recent experiment conducted in SPF chickens vaccinated on the first day of life with the vector rHVT-HA5 vaccine and challenged 4 weeks later with a H5N8 HPAIV isolated in Germany end of 2014 demonstrated 100% protection with significant reduction of shedding ( #24 ). The HPAIV used for challenge showed close to 98% amino acid similarity with H5N8 and H5N2 HPAIV isolated in 2015 in the USA.
A recent independent paper from FAO / Egyptian NLQP compares several vaccination regimes in front of a clade 2.2.1 H5N1 challenge: Vectormune AIAvian Influenza Reverse genetics Re-5, and a prime-boost regime Vectormune AI at day 1 plus Re-5 at day 8. Interestingly, conclusion states that Vectormune + Re-5 gave the best results ( #26 ) . Despite, it describes that Vectormune AI alone provided very good results, there was no statistical analysis for comparison.