Pertussis vaccine effectiveness and the California Pertussis Outbreak

Background: Pertussis declined sharply with the advent of whole cell vaccines and had fallen to very low levels by the 1970s.  In the 1990s, safety concerns prompted a switch from whole cell to acellular pertussis vaccines. Roughly coincident with this shift in vaccine type, pertussis incidence has been steadily increasing, and in 2010, California experienced its largest outbreak in over 50 years.  We identified factors related to vaccination that contributed to the California pertussis outbreak.

Methods:  We performed 3 separate case-control studies among members of Kaiser Permanente Northern California (KPNC).  Pertussis cases were identified on the basis of a positive polymerase chain reaction (PCR) test from 2006 to 2010.. We examined the association of pertussis with 1) whether a child had up-to-date immunization with the primary DTaP series; 2) the amount of time elapsed since receipt of the last (5^th) DTaP dose; and 3) whether a person received the Tdap booster vaccine.  Two sets of controls were used:  persons with a negative PCR test, and persons from the general KPNC population.  Conditional logistic regression was used to estimate the odds of a positive pertussis test as a function of vaccination history after adjusting for calendar time, age, sex, race, and medical facility. We also conducted a descriptive ecological analysis that juxtaposed pertussis incidence by age with the vaccine type in use at KPNC when persons that age were infants.

Results: Incomplete or delayed DTaP coverage put children at higher risk of pertussis (OR=2.62; 95% CI: 1.83-3.75). The effectiveness of the 5^th DTaP dose waned considerably over time (the odds of a positive pertussis test increased 33% with each year after vaccination [95% CI: 11-58%]). Tdap vaccination was moderately (50%) effective against pertussis (95% CI: 37-61%).  The pattern of pertussis incidence by age closely paralleled the age-related pattern of acellular vaccine use during infancy.

Conclusions:  Use of acellular pertussis vaccines likely played a key role in the recent California pertussis outbreak. Acellular vaccines appeared to provide less protection than  whole cell vaccines they replaced and their effectiveness may have waned substantially over time. Tdap was moderately effective in boosting protection.