NJ Savill et al. Nature (2006) Vol.442 p 757
“Silent spread of H5N1 in vaccinated poultry”
The first reports of the effectiveness of poultry vaccine indicated high levels of protection from disease and high levels of disease containment.
Doubts remained about the widespread use of vaccination:
o It would not be possible to distinguish vaccinated from diseased birds by blood test.
o A diseased bird would present a risk of infection to others.
o A vaccinated bird may still be vulnerable to disease.
o Low levels of disease would be harder to detect.
The present theoretical (computer modelling) research was designed to test whether vaccinated flocks could spread H5N1 disease.
90% of a flock would need to be vaccinated before the risk of disease propagation was reduced to 50%. Highly vaccinated flocks had a small probability (less than 10% chance) of transmitting disease to healthy but unvaccinated flock, the reason being that diseased birds in the vaccinated flock would be harder to discover; there being so few of them.
The authors also found that including unvaccinated birds within the flock increased the probability of detecting the disease but even this strategy was not fail-safe.
Vaccination of flocks usually aims to cover 90% of birds. The research suggests that rates above 95% are needed to prevent disease propagation. High rates of vaccination imply greater management costs. Vaccination does not guarantee protection of third party property or health.