SAN DIEGO—Patients who received an entirely acellular pertussis vaccine series had a “markedly increased risk of disease” compared with those receiving even one dose of whole-cell pertussis vaccine, investigators reported during IDWeek 2012.

Using the coincidence of the largest U.S. outbreak of clinical pertussis in 50 years with the end of U.S. availability of whole-cell pertussis vaccine, Maxwell Witt, research assistant from the Kaiser Permanente Medical Center, San Rafael, CA, and colleagues examined whether prior vaccination with whole-cell pertussis vaccine at any point in the vaccine series provided “superior protection” to those only vaccinated with acellular pertussis vaccine.

The investigators retrieved Kaiser Permanente electronic records for 465,059 persons from among a population of 3.2 million Northern California members who were 8 to 20 years of age as of May 15, 2012. In addition to type of pertussis vaccine administered, all persons with a positive PCR for pertussis between January 1, 2006 and November 15, 2011 were identified. This age group was examined as they had varying regimens of acellular and whole cell pertussis vaccines administered in their vaccines series.

A total of 253,826 persons had received all pertussis vaccines at Kaiser Permanente; of these, 158,027 received <5 total doses.

Among 31,686 members receiving 5 total doses of only acellular pertussis vaccine, 249 cases of pertussis occurred, for an attack rate of 786.1 per 100,000 person-years. Among 13,084 members who had received one or more doses of whole-cell vaccine as part of their vaccine series, the attack rate was 91.7 per 100,000 (relative risk [RR] 8.57 acellular only vs. at least one dose of whole-cell vaccine).

For those who received 6 doses, there were 134 cases of pertussis among 35,495 members, for an attack rate of 377.5 per 100,000 vs. an attack rate of 106.3 per 100,000 for the 15 cases among 14,106 who had received at least one dose of whole-cell vaccine (RR 3.55).

Witt noted that the risk of disease associated with an entirely acellular pertussis vaccine series “was mitigated, but not eliminated, by the presence of a sixth dose of pertussis vaccine (Tdap).” He added that recent pertussis outbreaks in the U.S. had peak attack rates among those who are in these exact age groups solely vaccinated with acellular pertussis vaccine. Witt concluded, “to prevent these outbreaks, earlier and/or additional booster doses are necessary.”