How does it work?
Essentially the way it works is that those that can resist the diseases that come, live to become breeders, the ones that are not do not survive and as a result of course do not breed or get passed on to someone else. When starting a flock and a disease hits you may lose many many birds, sometimes birds you are unusually attached to. This is much harder and harsher then just throwing some vaccination at them. You may lose the ability to pass on your eggs to others for an extended period and you certianly do not want to pass on sick birds to others. This is a harsh road to hoe but I believe it is worth it.
Why did I choose this?
According to the research I have done on the subject, most though not all chicken vaccines cause what is called a "carrier state" in the birds that are vaccinated. . This also happens when the birds catch and recover from these diseases naturally. Some though not all of these vaccines MUST be given in the first days of life to be effective and cannot be given later. The problem with this is that if a bird is strong enough to recover from a naturally caught full blown disease then it has a measure of resistance to that disease that they will pass on to their young, if a bird is vaccinated it will survive whether or not it is actually naturally resistant. Some hatcheries routinely vaccinate against different things, some do not. Most private people do not vaccinate and only some breeders for some things. Additionally on some vaccinations (unlike dogs and cats) you cannot "go back" and get a vaccination for that particular disease even if you wanted to because the window has passed to do so. As long as non vaccination is the norm generally, there are several issues of concern with vaccinated birds.
First they are carriers and can infect any bird they come in contact with that is not resistant or vaccinated against the disease. This means birds you own or chicks you hatch that were not vaccinated, this means other birds at a bird gathering like a show that are not resistant or vaccinated. If you bring home a vaccinated bird to an unvaccinated flock you are asking for trouble somewhere along the line. Carriers do not shed the germs all the time but off and on, eventually you could have on your hands a full blown infection of your whole flock, possibly high losses, medication costs, extra trouble, where you might not have otherwise.
It is unknown on any particular bird if it is actually resistant to any given disease unless they throw it off during an full blown flock infection or catch it and survive it but vaccinating it will cause it to survive to produce young regardless. If the parent is not resistant chances are high that the young will also not be resistant. If you breed that vaccinated bird you bought and do not vaccinate the young, it is highly possible that those young will catch the disease from the parent birds sooner or later and possibly die and possibly take any other unvaccinated birds with no resistance with them.
I choose to breed for resistance because I show sometimes and would hate to have my birds catch something from the vaccinated bird next to it or pass something due to vaccination to another bird if I can help it. Also I sell birds to all kinds of people, other breeders, people that just want pets, etc. My birds may go to live with vaccinated or unvaccinated birds and I want them to be able as best as they can to throw off diseases shed off by vaccinated birds or natural infection and not cause infection in someone elses flock, and if bred produce babies that are likely to resist diseases as well.
My birds will not be resistant to ALL diseases, only the ones they encounter here where they live, different locations can have different diseases that are a problem and there are other diseases which are pretty universal. With California being a place just infested with many of the poultry diseases all over, my birds will be good for anyone living here and have more then enough protection for many things anywhere else. Mind you, resistance is not neccisarily immunity though at it's best it essentially is immunity. Even resistant birds can and do sometimes catch the diseases depending on their level of resistance (which can vary among even resistant birds) but the goal is that a bird/flock is so resistant that when confronted with a given disease that it/they throw if off and never actually catch it in the first place. Problem solved in the most ultimate way and with no carrier state involved.
My birds are Marek's and Coryza resistant as I have had both these diseases directly hit me in the past and eliminate those birds that are not resistant. I am unsure of what else they are also throwing off because if it isn't actually taking birds or making them ill then I don't know it has passed through the flock and been resisted.
Situations that support vaccinated birds well in my opinion
If you don't breed or vaccinate all the babies you do breed, if you don't show, if all you have is a yard flock that stays in the yard/house. If all your birds are vaccinated then they cannot infect each other even though they shed the virus'. If you are not producing babies and then not vaccinating them then there is no chance of losing babies to shed virus' (be sure to inform those you pass babies on to that they are vaccinated and against what). This works well in a pet situation and there is no reason not to vaccinate or get vaccinated birds.
It is my opinion and the opinion of many others.....
If you have a vaccinated flock, don't bring in unvaccinated birds, if you have an unvaccinated flock you are asking for problems by bringing in birds that have been vaccinated with vaccines that cause a carrier state.