>>13594669https://journals.plos.org/plosbiology/article/info:doi/10.1371/journal.pbio.1002198#sec001This graph comes from them introducing mild strains and lethal strains into chickens that didn't have the same genetic history as farm birds. The dotted lines is the survival rate of unvaxxed chickens. In a separate experiment, they go on and introduce sentinel chickens which are not vaccinated into test populations to see what happens. Sentinel chickens don't get infected when birds are unvaccinated because viral loads are very low and the unvaccinated birds die before it spread. Birds born from vaccinated mothers on the other hand, even those that die a few days later than unvaxxed, shed much more virus. These chickens were not kept in an environment much different than one would have if they were laying eggs in someones backyard except for lack of sunlight and maybe feed variety.
I am looking for studies where unvaccinated chickens were isolated after exposures to the virus. The anticipated result is virulence will stay the same. This seems like a reasonable conclusion for the death strains where death occurs before transmission to nearby birds, but I am not very convinced with the milder variants, while they are still horribly lethal by any virulence standard.
The other thing I am thinking about is how the evolution of MVD was tracked from the introduction of the vaccine onward. It seems strange that a virus would adapt to become more lethal without some factor making it more prevalent or preferable within the infected and vaccinated chicken itself. Does the virus spread outside of its original channels to the rest of the bird just through constant unmitigated exposure. As can be seen in vaccinated birds, whatever the virus is doing, it is serving to spread itself more and
more.As a kind of corollary thought, I wonder if there is some symbiotic relationship developing in the vaccinated.
