The effectiveness of diphtheria, tetanus, and acellular pertussis (DTaP) vaccines wanes substantially after the 5thdose given at ages 4-6years, but has not been described following 5 doses of the same type of DTaP vaccine. We investigated waning effectiveness against pertussis in California over nearly 10years, which included large pertussis outbreaks, following 5 doses of GSK DTaP vaccines (DTaP3). We conducted a case-control study (NCT02447978) of children who received 5 doses of DTaP at Kaiser Permanente Northern California from 01/2006 through 03/2015. We compared time since the 5thdose in confirmed pertussis polymerase chain reaction (PCR)-positive cases with pertussis PCR-negative controls. We used logistic regression adjusted for calendar time, age, sex, race, and service area to estimate the effect of time since the 5thDTaP dose on the odds of pertussis. Our primary analysis evaluated waning after 5 doses of DTaP3. We also examined waning after 5 doses of any type of DTaP vaccines. Our primary analysis compared 340 pertussis cases diagnosed at ages 4-12years with 3841 controls. The any DTaP analysis compared 462 pertussis cases with 5649 controls. The majority of all DTaP doses in the study population were DTaP3(86.8%). Children who were more remote from their 5thdose were less protected than were children whose 5thdose was more recent; the adjusted odds of pertussis increased by 1.27 per year (95% CI 1.10, 1.46) after 5 doses of DTaP3and by 1.30 per year (95% CI 1.15, 1.46) after any 5 DTaP vaccines doses. Waning protection after DTaP3was similar to that following 5 doses of any type of DTaP vaccines. This finding is not unexpected as most of the DTaP vaccines administered were DTaP3.Following 5 doses of DTaP3vaccines, protection from pertussis waned 27% per year on average. NCT number: NCT02447978.