You must wait until your puppy has had all his shots plus two weeks for full immunity. Puppies get maternal antibodies from nursing which stay in their bodies for weeks after weaning and they interfere with the early shots.
A study of a cross section of different puppies showed that the age at which they were able to respond to a vaccine and develop protection (become immunized) covered a wide period of time. At six weeks of age, 25% of the puppies could be immunized. At 9 weeks of age, 40% of the puppies were able to respond to the vaccine. The number increased to 60% by 16 weeks of age, and by 18 weeks, 95% of the puppies were protected by the vaccine. Vaccination Recommendations for Puppies (Puppy Shots)
Unless your backyard is fenced in, it isn't even safe. The parvo virus is shed in the feces of infected dogs. We had a puppy on another forum get parvo from his own backyard because neighborhood dogs were using it for a bathroom.
