A. If you are walking in a suburban area and avoiding contact with dogs you don't know, your puppy should be fine. We don't recommend walking on city streets at this age because of the higher proportion of unvaccinated dogs in urban areas. And definitely don't visit dog parks or allow nose-to-nose contact with any dog you don't know to be vaccinated and healthy.
After she has had her final booster at 4 months of age she is considered well protected from contagious diseases. The reason for this is that during the initial vaccination series, a puppy still has maternal antibodies - protection from disease that she got through the placenta from her mother. It's good protection for her, but can cause vaccinations to be less effective. We know that by 4 months of age, the maternal antibodies are gone, so the last vaccination in the puppy series (and kitten series as well) should be given at this age.
Which brings up the question of why animals need annual or triennial vaccinations after the puppy or kitten series. People don't need yearly boosters, except for the flu shot, but that's because that virus is constantly changing. The answer to the question is that we don't actually know how long the immunity to any given vaccine lasts for dogs and cats. The companies that make vaccines have done studies to prove that immunity is effective for at least a year (after the initial series). And some new vaccines have been shown to be effective for three years. But beyond that, many factors come into play, such as the animal's individual immune system, the type of vaccines they had as a puppy or kitten, and their level of exposure to diseases. So we err on the side of caution and recommend the yearly vaccination.
--Dr. Heidi Strand is a veterinarian for the East Bay SPCA in Dublin. She has lived in the Tri-Valley for 10 years with her family and an assortment of four-legged friends. Questions can be mailed to 315 Diablo Road, Suite 100, Danville 94526; or e-mailed to firstname.lastname@example.org. Her column runs every other week.