How to cite this article: Vet. Res. (2009) 40:15
DOI: 10.1051/vetres:2008053

Risk factors and characteristics of H5N1 Highly Pathogenic Avian Influenza (HPAI) post-vaccination outbreaks

Joerg Henning¹, Dirk U. Pfeiffer² and Le Tri Vu<sup>3

1</sup>  School of Veterinary Science, University of Queensland, St. Lucia, Queensland, Australia
²  Epidemiology Division, Department of Veterinary Clinical Sciences, The Royal Veterinary College, University of London, United Kingdom
³  Regional Animal Health Centre VI, Ho Chi Minh City, Viet Nam

Received 6 October 2008; accepted 10 December 2008; published online 12 December 2008

Abstract - Highly pathogenic avian influenza (HPAI) virus H5N1 is now endemic in South-East Asia but HPAI control methods differ between countries. A widespread HPAI vaccination campaign that started at the end of 2005 in Viet Nam resulted in the cessation of poultry and human cases, but in 2006/2007 severe HPAI outbreaks re-emerged. In this study we investigated the pattern of this first post-vaccination epidemic in southern Viet Nam identifying a spatio-temporal cluster of outbreak occurrence and estimating spatially smoothed incidence rates of HPAI. Spatial risk factors associated with HPAI occurrence were identified. Medium-level poultry density resulted in an increased outbreak risk (Odds ratio (OR) = 5.4, 95% confidence interval (CI): 1.6–18.9) but also climatevegetation factors played an important role: medium-level normalised difference vegetation indices during the rainy season from May to October were associated with higher risk of HPAI outbreaks (OR = 3.7, 95% CI: 1.7–8.1), probably because temporal flooding might have provided suitable conditions for the re-emergence of HPAI by expanding the virus distribution in the environment and by enlarging areas of possible contacts between domestic waterfowl and wild birds. On the other hand, several agricultural production factors, such as sweet potatoes yield, increased buffalo density, as well as increased electricity supply were associated with decreased risk of HPAI outbreaks. This illustrates that preventive control measures for HPAI should include a promotion of low-risk agricultural management practices as well as improvement of the infrastructure in village households. Improved HPAI vaccination efforts and coverage should focus on medium poultry density areas and on the pre-monsoon time period.

Key words: avian influenza / H5N1 / poultry / risk factor / HPAI outbreak

Corresponding author: j.henning@up.edu.au

© INRA, EDP Sciences 2009