A booster dose is given annually. Viral infections in particular suppress the immune system so it can't respond fully to the vaccine. If the pups are already infected with a Parvovirus or distemper virus, vaccines aren't going to be effective against a disease that is already incubating and suppressing the immune system. Malnutrition or parasitism will also suppress the immune system. Some vaccines may appear cheaper price-wise and seem to include the same organisms but if it doesn't give sufficient protection when the dog is exposed to the disease or worse, induces disease, you've just thrown your money away.
Prevention is easier and less costly than treatment any day.
1999 and Pfizer Animal Health, New York.