I've never had a vaccinated chicken in the past 40 yrs and never had illness in any of the flocks either. I also don't stress sanitation as other people see sanitation~bleaching out coops, biosecurity measures, cleaning out the feces every day, etc......the best way to keep chickens clean and healthy is to free range them on plenty of land so that soils are not over impacted by their presence.
If one cannot do that, the next best thing is a cultured deep litter in the coop and run. Barren, compacted runs that cannot absorb moisture, have too much nitrogen in the soil due to fecal loads and have no good habitat for beneficial bacteria and molds will grow unhealthy loads of pathogens and parasites. Providing a deep litter that mimics a forest floor in this area can keep soils loose and draining well, will give the nitrogen some carbonaceous materials with which to bind, provides healthy soils that can harbor the beneficial microbial life that can keep the less beneficial microbes in check.
Giving chicks a hunk of sod from where they will be living, right in their brooder from day one, can provide them exposure to the germs in their eventual environment at the right time in their lives~the first few weeks when their immune system can best create antibodies against these natural environmental microbes.
The worst mistake most people make is to keep chicks in an inside brooder, all clean and separate from these germs until they are around 2 mo. old and then they turn them out on the soils....it's no coincidence that 2 mo. of age is when most backyard flocks contract coccidiosis, whether they have had medicated feeds or not. They are totally unprepared and vulnerable to these germs because no one has developed their immune systems as they would have been if raised by a broody hen in their natural environment. I've lived a long, long time now with chickens and have yet to see a single chick raised by a broody to have pasty bottom or any kind of illness. One reason is that they are exposed to the mother's germs and have formed the correct antibodies for their environment.
I don't care if I were to ever have a whole flock wiped out by an illness, I'd still not vaccinate or feed medicated feeds or even give meds of any kind. If this were to happen it means I've failed in my flock care along the way by not building good immune systems, not culling the flock to remove birds that are not naturally hardy or are too old to lay, haven't chosen the correct breed/genetics, and not providing healthy habitat for their living. At that time I'd go back to square one and work harder to remedy those things.