I was looking to see if there was parvo rate info for your area (like where I am had a big parvo outbreak a year ago) and saw this interesting, slightly OT article:
Yellowstone wolf population dips / Parvovirus blamed for steep decline - SFGate
I am parvonoid and do not want a foster puppy exposed. I carry them into the vet office, try to change shoes at the door at home, keep them in the yard, and even limit my own dogs' outings to minimize risk. Even with everything it is still a risk.
There are things you can do to increase socialization - having people over, finding people with well behaved, puppy-friendly adult dogs or other vaccinated puppies, etc. Have the puppy exposed to noises, surfaces, things that they would see in the outside world.
The AVMA has a position paper on this, stating that behavior issues from lack of socialization result in dogs being put to sleep, but while I agree this is a vital period, if a dog has good temperament, owners who are committed to the dog and training, the few weeks to get that coverage, TO ME, should not result in such drastic behaviors.
I've yet to see it.
I think maternal antibody info is confusing but some is here: Parvovirus: Serious Diarrhea in Puppies & Dogs
and the Mar Vista vet site also has a big thing on Parvo.