Why does a puppy need more vaccination?

Good question! The reason is that no one can be sure that the pup will actually mount a good antibody response to the disease just from one vaccination. The age of the pup and just how much immunity it has received from its mother will complicate the "probability of protection". So... if the pup has lots of immunity (called PASSIVE IMMUNITY) that was borrowed from the mother during early nursing, this immunity will actually interfere with the pup's ability to make its own immunity from the vaccine challenge. The idea is to get the vaccine into the pup just as soon as the mother's passive immunity wears off (NOW the pup is very susceptible to sickness if it is exposed to the disease!) so that the pup can make more lasting immunity of its own. The precise time when a pup can respond well to avaccine is variable... it might occur at 6 weeks of age or might be 12 weeks. So to be as safe as possible, why not start vaccinating at 6 weeks and end at 16 weeks? Almost 99 per cent of puppies will develop a good immune level to the various disease from a vaccine schedule similar to the one above.