It sounds like they're fully vaccinated and up to date, so that's really all you can do. If it's a different strain, being vaccinated isn't going to cause harm. But unfortunately there's nothing you can do to stop it. I would talk to your local vets and see if they have any details on vaccinated dogs becoming ill....Or whether it is a different strain. I've had adult, vaccinated dogs around parvo pups and have never had an issue. It was never a worry.