One of the common poultry vaccines in the US is for Marek’s disease, which has arguably been a disaster. Over time, the disease has evolved to be more virulent

One of the common poultry vaccines in the US is for Marek’s disease, which has arguably been a disaster. Over time, the disease has evolved to be more virulent so that it can spread in vaccinated flocks. As a side effect, its mortality rate in unvaccinated flocks has been increasing, and some strains of it are approaching 100%. Normally, such a contagion would burn itself out instead of spreading. The mortality rate in vaccinated flocks is non-trivial and also increasing. I think it’s still lower than the unvaccinated mortality rate before the vaccine was introduced, but I’m not sure. (I am by no means an anti-vaxxer; the currently available human vaccines are effective and safe, as is the Marek’s vaccine. Marek disease’s is a worst case scenario outlier, and enabled by the conditions in modern poultry farms, which are atrocious.)