Evaluation of the vaccination efficacy against H5N1 in domestic poultry in the Red River Delta in Vietnam

Abstract

SUMMARYThe domestic poultry population in Vietnam has been vaccinated against highly pathogenic avian influenza (HPAI) H5N1 since 2005. Since then, outbreaks have continued to occur without a clear understanding of the mechanisms involved. The general objective of this study was to understand the epidemiology of the disease in the context of vaccination and to draw some conclusions about vaccination efficacy in the domestic poultry population of the Red River Delta area. Five cross-sectional surveys to measure the serological and virological prevalence in vaccinated and unvaccinated poultry were performed from the end of 2008 to June 2010. The global seroprevalence was 24 % (95 % confidence interval 19 . 9-28 . 2). Determinants of vaccine immunogenicity were identified separately in chickens and ducks as well as determinants of the seroconversion in unvaccinated birds. The results highlight the difficulties in maintaining good flock immunity in poultry populations using inactivated vaccine in the field with two vaccination rounds per year, and in preventing circulation of virus in co-existing unvaccinated poultry.

Landscape attributes driving avian influenza virus circulation in the Lake Alaotra region of Madagascar

“…This is consistent with findings from previous studies which showed that the risk of HPAI H5N1 increased with flock size (Desvaux et al, 2012). This may be due to an increased transmission in large flocks, related to higher poultry density and higher degree of contacts between birds.…”

“…In spite of these limitations, the present study made it possible to identify risk factors associated with exposure of poultry to AIV. The study found higher AIV seroprevalence in palmipeds (ducks and geese) than in chickens, which is consistent with findings by other serological surveys carried out in unvaccinated birds raised in small-scale farms of Indonesia (Santhia et al, 2009) or Vietnam (Henning et al, 2009;Desvaux et al, 2012). The reason why this should be so can be explained.…”

“…First, ducks and geese serve mainly as egg producers and have a long lifespan and may thus have been exposed to AIV for longer periods than chickens, which are generally slaughtered at the age of 6 to 12 months. Desvaux et al (2012) have shown that the effect of species on H5 seroprevalence persist after being adjusted for poultry age, and this indicates that age was not the only explanation for the species variation observed. Second, experimental studies suggest that ducks shed virus for longer periods of time and at higher than chickens (Achenbach and Bowen, 2011;Arsnoe et al, 2011;Hénaux and Samuel, 2011) resulting in a greater probability of detection of specific antibodies in ducks.…”

Role of vaccination-induced immunity and antigenic distance in the transmission dynamics of highly pathogenic avian influenza H5N1

“…These studies show that use of vaccines that are antigenically distant to the circulating strains led to lack of protection due to low HI titres against the challenge strains. Upon closer examination however, it can be seen (where the data are available) that even HI titres against the vaccine strains were low in these vaccination programmes, thus indicating that the vaccine had failed to induce immunity to begin with [20,37,38]. In addition, reports from the field have emerged in which it is shown that in some cases where vaccination against HPAI is applied nationwide, the population coverage is extremely low, thus allowing transmission to take place in a seemingly vaccinated population [12,14,20].…”

Risks of Avian Influenza Transmission in Areas of Intensive Free-Ranging Duck Production with Wild Waterfowl

“…Prevention measures aimed at reducing the intensity of AIV transmission should be focused on these areas. Successful vaccination programs may be difficult to sustain in the long-term and should be complemented by other control measures (Desvaux et al 2013). Our synthesis highlights the importance of free-grazing duck farming in increasing risks at the wild-domestic interface.…”