The main reason why dog vaccines need to be administered in two doses is to ensure that the immune system is fully protected against the disease. The first vaccination can stimulate your dog's immune system to start producing antibodies, but these antibodies may not be enough to provide long-term protection. The second vaccination can further activate the immune system to produce more antibodies and strengthen the previous immune response to ensure that the dog obtains sufficient immunity.
In addition, some vaccines need to establish long-term protection in the dog's immune system, which means they need Repeat the vaccination within a certain period of time. For example, the rabies vaccine usually requires annual vaccination to maintain immunity. Therefore, the time between vaccinations and the schedule for repeat vaccinations may vary depending on the type of vaccine and the age of the dog. If you have questions about your dog's vaccination schedule, it's best to consult a veterinarian or pet medical professional.