Protective Efficacy of Poultry Vaccines against Recently Circulating Highly Pathogenic Avian Influenza (HPAI) H5N1 Virus Isolates from Markets and Farms in Hong Kong 2008

Abstract

Introduction: Highly pathogenic avian influenza (HPAI) H5N1 remains a major threat to animal and public health. Since 2003, Hong Kong has successfully used poultry vaccination as part of its strategy to minimise this threat within Hong Kong. In mid-2008, an HPAI H5N1 outbreak occurred in a vaccinated poultry farm in Hong Kong.

Aims: a) to compare protective efficacy of different poultry vaccines against the 2008 farm outbreak strain; and b) to assess whether there needs to be a change in the poultry vaccine used in Hong Kong. Methods: White leghorn chickens were raised in a clean laboratory environment and divided in four groups: a) unvaccinated controls; or vaccinated with b) NoblisR vaccine (the vaccine in use in Hong Kong); c) PoulvacR vaccine, or d) Harbin Re-5R vaccine, in accordance with the manufacturer’s recommendations. Pre-challenge sera were collected at day 63 of age. Then, 9-12 chicken from each group were challenged by infection with either A/chicken/Hong Kong/8852-2/08 (H5N1), a clade 2.3.4 virus isolated from the affected farm or with A/chicken/Hong Kong/782/2009 (H5N1) (clade 2.3.2). Experimental challenge was done with a dose of 105 egg-infection-dose50 in a volume of 0.5 mL by the intranasal route. Haemagglutination inhibition (HI) tests were used to evaluate the antibody responses in vaccinated chicken post-immunisation.

Results: All three vaccines provided good protection from death and significantly reduced virus shedding following challenge with A/chicken/Hong Kong/782/2009. Only Harbin Re5 vaccine provided protection against challenge with A/chicken/Hong Kong/8852-2/08, the strain associated with the farm outbreak. Sera from vaccinated chickens had lower geometric HI titres against A/chicken/Hong Kong/8825.2/08, as compared to two other clade 2.3.4 and one clade 0 virus. Alignment of amino-acid sequences of the haemagglutinin of A/chicken/Hong Kong/8825.2/08 and the other H5 viruses revealed several mutations which may correlate with loss of vaccine protection.

Conclusions: Our results indicated that some clade 2.3.4 HPAI H5N1 viruses have undergone antigenic changes that allow them to evade immunity from poultry vaccines. The NobilisR vaccine then in use in Hong Kong did not provide acceptable protection against some circulating H5N1 virus strains. A change in the poultry vaccine being used in Hong Kong needs to be considered. The findings highlighted the need for ongoing surveillance and monitoring of vaccine-induced immunity against currently circulating virus strains by serological tests supplemented where necessary with experimental vaccine challenge studies in chicken. Parallel Session 2: S7