more Here.
The implications are frightening, since many of the vaccines we use are "attenuated", i.e. mild versions of the lethal viruses: If you give the chickens both vaccines for this infection (a form of chest cold in chickens) the viruses combine to form a lethal strain.
But of course you usually don't give two different vaccines to the same bird for the same disease, so what happened? And here is the problem:the attenuated virus spread in a normal fashion to unvaccinated birds:
Given that farmers generally do not use both vaccines on the same animal, the recombination suggests that the vaccine strain first spread to a non-vaccinated population before moving into an animal population vaccinated with a different strain.
This has implications in humans, since some vaccines (e.g. MMR, nasally given flu vaccine) are live virus vaccines...
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