Potential risks of vaccination against avian flu epidemics

Highly pathogenic H5N1 influenza A viruses have spread relentlessly across the globe since 2003. They are associated with widespread death of poultry, substantial economic loss to farmers, and reported infections of more than 300 people with a mortality rate of 60%. Influenza prevention and containment strategies can be considered under the broad categories of antiviral, vaccine, and non-pharmaceutical measures. In particular, using vaccination to reduce the transmission rate might provide an alternative to mass culling by reducing both the susceptibility of healthy birds and the infectiousness of infected birds. However, although vaccination can be a useful tool for control of avian influenza epidemics, it might engender the emergence of a vaccine-resistant strain. Field and experimental studies show thatsome avian influenza strains acquire resistance against vaccination. We investigated, in the context of the emergence of a vaccine-resistant strain, whether a vaccination program can prevent the spread of infectious disease. Our main findings are that such a program might lead to an emergence and replacement of the vaccine-resistant strain over a large geographical region, and that a vaccination to prevent the spread of disease can instead spread the disease. Thus, if the vaccinations are not used appropriately, prevention and control will be negatively affected by the vaccination program. Further, from our theoretical studies, we propose how a vaccination against avian influenza should be used.