Vaccination reduced the incidence of outbreaks of low pathogenicity avian influenza in northern Italy.

When outbreaks of avian influenza (AI) occur in poultry populations, the main goal to achieve is the control and eradication of the infection. However, quantitative information on risk factors for AI spread and efficacy of AI control measures such as vaccination in the field is limited. From 2000 to 2005, H5 and H7 low pathogenicity (LP) AI viruses caused four epidemics in poultry in northeastern Italy. Italian veterinary authorities implemented emergency vaccination in the 2000-2001 and 2002-2003 LPAI epidemics and prophylactic vaccination from July 2004. The aim of this study is to quantitatively evaluate the efficacy of AI vaccination in the field, taking into account the different strategies (emergency and prophylactic) implemented. Moreover, risk factors for LPAI spread in domestic poultry were studied. By survival analysis, we observed a two-fold increase in survival probability for vaccinated poultry farms compared to unvaccinated ones. In meat turkeys, vaccination protocols changed in the different epidemics, and a relationship between protection and the number of vaccinations was observed; two or three vaccine administrations protected flocks from LPAI, whilst four administrations did not significantly reduce the risk of infection. In meat turkeys the risk of AI infection increased also with the increase in both farm size and proximity to an infected farm. In general, we observed a lower number of outbreaks and a faster eradication of the infection when LPAI viruses introduced in a preventively vaccinated poultry population. This study provides insights on LPAI vaccination efficacy and on risk factors involved in LPAI infection at farm level. To our knowledge, this is the first study which quantitatively evaluates AI vaccination efficacy and compares different vaccination strategies and protocols using field data.