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A retrospective, single-center chart review of the 2010 Bordetella pertussis outbreak in California found that a time interval greater than 3 year since vaccination with acellular pertussis (aP) correlated with increased risk for acquiring the disease. Research assistant Maxwell Witt, Kaiser Permanente Medical Center, San Rafael, California, USA, reported that children between 8 and 12 years had higher attack rates and reduced vaccine effectiveness compared with children aged 2 to 7 and 13 to 18 years, possibly a reflection of greater time since their last aP dose.

Since the replacement of whole-cell pertussis vaccine with the better-tolerated aP version in 2002, questions regarding its efficacy and durability have lingered [Zhang L et al. Cochrane Database Syst Rev 2011]. Researchers at San Rafael Kaiser Permanente (KP) Medical Center, led by David Witt, MD, saw the California outbreak as an opportunity to observe aP vaccine performance by age, time since last vaccine, and vaccine status.

Between March and October 2010, patients who presented to the San Rafael KP pediatrics department with a severe cough for greater than 1 week and a positive PCR for B. pertussis were considered infected and included in the review. Electronic medical records were examined for demographic information and vaccine status.

In all, 132 patients <18 years were included. Vaccination status among children aged ≤12 years at presentation revealed that 85% were fully vaccinated, 7% was under vaccinated, and 8% was unvaccinated (never vaccinated). B. pertussis attack rates were shown to be highest among 8- to 12-year olds, compared with 2- to 7- and 13- to 18-year olds (p=0.002, one sample t-test; Table 1). Among children <12 years, a trend toward lower attack rates among fully immunized children versus under- or never-immunized children was observed, but the difference was not statistically significant. In contrast, children aged 13 to 18 years who were not fully immunized had significantly higher attack rates compared with other age groups (p=0.009). No patients in the cohort were hospitalized or died from their illness.

Vaccine effectiveness, a metric of the field performance of the vaccine, was calculated by comparing attack rates between under- and never-immunized versus fully immunized patient groups (of note, effectiveness should not be confused with efficacy, which reflects performance in a prospective placebo-controlled trial). The effectiveness of aP varied by age group: 41% (95% CI, 21% to 54%) and 79% (95% CI, 73% to 84%) within the 2- to 7- and 13- to 18-year olds, respectively, possibly reflecting more recent immunization, but only 24% (95% CI, 0% to 40%) in the 8- to 12- year old age group.

The authors concluded that aP is highly effective within 3 years of administration after which its protection may diminish. Should larger studies confirm these findings, additional scheduled dosing or targeted vaccine programs during outbreaks may be proposed. One attendee, however, challenged the relevance of the findings including the use of the phrase “vaccine failure,” arguing that strict case definitions had not been used. In Dr. Witt's opinion, B. pertussis carriage in the face of a viral illness had not been ruled out and therefore, these results cannot be used to question the efficacy of the vaccine.

• © 2011 MD Conference Express