"Unvaccinated dogs" refers to dogs that have not received vaccines, which are medical shots that help protect against certain diseases or illnesses. Full definition
In the USA between 1971 and 1973 629 cases of rabies in dogs were reported and 90 % of these occurred in unvaccinated dogs. (thehappypuppysite.com)
A study on Lyme Disease in dogs in Connecticut in 2005 showed similar proportions with 63 % of unvaccinated dogs becoming infected, compared with 25 % of vaccinated dogs. (thehappypuppysite.com)
I am comfortable giving 1 modified live vaccination for the diseases that I vaccinate for when a previously unvaccinated dog who is at least 16 weeks of age or older comes to the clinic. (vetinfo.com)