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Abstract
Knowledge of the relation between the antigen content of inactivated avian influenza
(AI) vaccines, the serological response after vaccination and protection of vaccinated
animals is important for the choice of optimal vaccines and vaccination regimes as
well as for the assessment of criteria for the licensing of new AI-vaccines. We studied
this relation in a dose response study using inactivated H7N7 avian influenza vaccines
with varying antigen content. The serological response depended on the antigen content
of the vaccines. Anti-AI antibodies were detected most frequently with ELISA, followed
by the virus neutralisation test and the haemagglutination inhibition (HI) assay.
Chickens with measurable HI-antibody titers, using homologous H7N7 antigen, were all
protected against clinical disease after challenge with highly pathogenic A/chicken/Netherlands/621557/03
H7N7 virus. However, in these chickens high levels of virus could still be present
on days 2-4 after challenge. The reduction of virus titers after challenge, depended
on the antigen content of the vaccines as well as on the serum antibody titers. While
10 haemagglutinating units (HAU), equivalent to 0.8 microg haemagglutinin (HA) protein,
per vaccine dose was sufficient for prevention of clinical disease, 128 HAU (9 microg
HA) per dose was required for reduction of virus titers in all chickens to 10(3) egg-infectious
dose 50% (EID(50)) or less. In order to reduce virus titers below 10(3)EID(50) per
swab a HI-antibody titer of 64 was required. After use of the vaccine with the highest
antigen content, challenge still induced a booster of antibody titers which is indicative
of replication of challenge virus.