Name the least interval between doses for minimum age being 1st dose at 6 weeks old for the diphtheria-pertussis-tetanus vaccine?
CREATE TABLE "routine_immunization_schedule_for_infant" (
  "vaccine" text,
  "minimum_age_at_1st_dose" text,
  "number_of_doses" text,
  "dose" text,
  "minimum_interval_between_doses" text,
  "route" text
);
SELECT "minimum_interval_between_doses" FROM "routine_immunization_schedule_for_infant" WHERE "minimum_age_at_1st_dose"='6 weeks old' AND "vaccine"='diphtheria-pertussis-tetanus vaccine';