An attenuated virus is a ‘live’ virus, meaning that if a bird were vaccinated with it then they would possibly give any unvaccinated birds the actual illness.
I think there is a ‘killed’ virus for Mareks but not 100 percent sure.
Flu and Covid vaccine is a ‘killed’ virus unless it’s an mRNA vaccine which are what most all vaccines are these days as they are faster and less expensive to create, and have far better efficacy.
There has been incidence of Polio outbreaks here in North America in persons vaccinated with attenuated Polio vaccine (which is not used here in NA at all), they then can possibly infect a person not vaccinated for Polio. And that is what has happened here in NA, children from 3rd world countries are vaccinated with the attenuated vaccine, they ‘infect’ someone who hasn’t been vaccinated or ever had Polio and then pass it on to anyone here who haven’t been vaccinated, usually children who are not vaccinated



poor wee things.
By_bob can speak to Mareks and other poultry vaccine more that I can - though I am thinking I should get myself educated on the subject. If this were horses we were talking about I could tell you everything about what they should get! But with my chickies I am sadly uninformed, there doesn’t seem to be that much info for backyard chickies

Those are beautiful eggs, lovely colours