Live vaccine is shed in the droppings - of animals, birds and humans
The polio vaccine is shed by babies - I as a mother had to have a 'booster' when my son had his first polio vaccine - to protect me. (that was from my doctor !!)
The rules in our combine are that birds given a live vaccine cannot mix with birds from other lofts for 21 days (and that is because the disease can be transmitted to non vaccinated birds)