2,4,6m dpt vac..then dpt infant booster dose can be taken until 2 y with same efficacy? or must be taken at 16 months and less effective after that?

DPT VACCINATIONS...: THE USUAL SCHEDULE IS BASED ON INITIAL PROTECTION ON A NON AS RESPONSIVE IMMUNE SYSTEM AND SUBSEQUENT DOSING BUILDING UP AN ANAMNESTIC REACTION- OR INCREASED REACTION THAT WILL GIVE A COVERAGE FOR YEARS TO COME...AFTER THE 6 MONTHS DOSE THE NEXT ONE IS USUALLY BETWEEN 18M TO 2 YEARS OLD AND ANOTHER ONE AROUND 5 YEARS OLD WITH A SUBSEQUENT ONE AROUND 15 YEARS OF AGE...