I don't worry about that stuff much, my dogs would be out with me anyway. And they haven't been vaccinated ever except for rabies. A new guy that is training with us has a 7 month old that he did have titered. Never vaccinated and he shows antibodies for Parvo, vet still wanted to vaccinate him
He wondered why, since he had obviously already encountered it.
Parvo is everywhere and can persist for years and years in the soil. If your dog is alive and about, it's in contact with it. Sure it can be devastating at times, and very often others it does nothing. A new strain could be scary, if it was very virulent and there wasn't much in the way of natural protection. But then how good is the old vaccine going to be anyway?