Live attenuated viral vaccines fool your immune system into believing you’ve come into contact with a real virus to stimulate an antibody response
When you get a live attenuated viral vaccine, you shed live vaccine strain virus in your body fluids—just like when you get a viral infection and shed virus in your body fluids
After getting a live virus vaccine, you can shed and transmit vaccine strain virus to other people
Live attenuated viral vaccines have the potential to affect the evolution of viruses, which are constantly recombining with each other, because vaccine strain live viruses are released into the environment where further mutations can occur
A healthy immune system is the most powerful way to resist infectious diseases or heal after infection and the efficient functioning of your immune system is dependent on healthy gut flora
Proof of this lies in the Sabine Oral Polio Vaccine. If there is a group of un-vaccinated people and one of them is given the vaccine, the others are at risk of developing real polio.
While that is technically correct, they are still able of transmitting genetic information to a second host and in some case may cause full blown disease.