Dose response effects of avian influenza (H7N7) vaccination of chickens: Serology, clinical protection and reduction of virus excretion

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 μg haemagglutinin (HA) protein, per vaccine dose was sufficient for prevention of clinical disease, 128 HAU (9 μg HA) per dose was required for reduction of virus titers in all chickens to 10³ egg-infectious dose 50% (EID₅₀) or less. In order to reduce virus titers below 10³ EID₅₀ 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.

Introduction

Avian influenza (AI) is a viral disease that can spread rapidly within and between poultry flocks. AI viruses are being subdivided in a series of serotypes based on the presence of different haemagglutinin (HA) and neuraminidase (NA) surface proteins. Most AI-viruses cause subclinical infection or only mild respiratory disease. However some AI-viruses of the H5 and H7 subtypes are highly pathogenic (HPAI) and can cause mortality in high percentages of infected chickens. These HPAI viruses can cause large outbreaks that are difficult to contain. Since the 1990s the number and severity of AI-outbreaks worldwide has increased markedly [1]. In Europe thee have been large outbreaks of H7N1 (Italy 1999) and H7N7 (The Netherlands 2003). The large continuing outbreak of H5N1 in Asia, which has spread to Africa and Europe, has emphasised that AI-outbreaks can have a major impact on animal health and economy, as well as on human health. Up to now approximately 250 persons have died because of H5N1 infections.

Next to quarantine measures, culling of infected poultry flocks, and improved biosecurity measures, vaccination is an important tool to prevent disease and to limit outbreaks. Ideally, vaccination should prevent infection. However, this is very difficult to achieve. A more realistic goal is the prevention of disease and reduction of virus excretion. For control of the disease it is essential that vaccination decreases virus excretion to levels that are insufficient for virus transmission within poultry flocks. It is therefore important that vaccines are being selected that are effective in decreasing virus excretion and that this is embodied in criteria that can be used for the licensing of AI-vaccines.

As a consequence, the factors that determine the efficacy of AI-vaccines must be studied systematically. Systematic data on the relations between vaccine antigen content, serological response and decrease of virus replication after challenge are lacking in literature. Dose response vaccination studies are very suitable to study vaccine efficacy, since the amount of vaccine antigen and height of the antibody response that is required for clinical protection and reduction of virus excretion, can be determined in these studies. It has been shown that the efficacy of inactivated AI poultry vaccines is determined by antigen content [2] as well as by antigenic similarity (homology) between vaccine virus and challenge virus [3], [4]. Furthermore, inactivated H7N3 and H7N1 AI vaccines can prevent clinical disease and virus transmission after challenge with HPAI H7N7 [5]. Here we present the results of our dose response vaccination study in which the vaccine antigen content and the induced antibody response are related to clinical protection as well as reduction of virus titers after challenge of vaccinated chickens.