Vaccination

What do you do when you find out that your child is playing with a non vaccinated child. So it freaks the hell at out me the thought of my child getting a disease any disease. Even though all my children have been vaccinated against every single thing that they can be. So I have this rule that my child will not play with your child unless they have been vaccinated against the main diseases. I never even accounted this problem until we moved to the USA. We used to live in England. I will actually ask parents if they child has had vaccinations. What happens when the disease comes back and it mutates. Then the vaccine doesn't work anymore. That scares me the most