BACKGROUND:
California’s pertussis incidence rate has been rising gradually since the mid-1970s with most cases still occurring in children. Vaccination is an effective way to prevent severe disease. However, cases most likely to have severe outcomes (very young infants) are too young to have completed the three-dose series.

OBJECTIVE:
To evaluate whether vaccination status plays a role in protecting against severe pertussis disease.

METHOD:
Pertussis is a reportable condition in California. Data collected include complications due to pertussis and vaccination history. Case investigation data from 1995-2003 were analyzed. Statistical differences were tested by chi-square analysis.

RESULT:
From 1995-2003, 2871 cases were reported in infants 0-6 months of age. 1469 were under 2 months of age and therefore too young to receive any vaccinations. Of the 1402 infants age 2-6 months old and therefore eligible for 1-3 doses of pertussis vaccine, 217 were age-appropriately immunized, and 1185 were unvaccinated or not age-appropriately immunized (i.e., were under-immunized). Of those 2-6 months of age, infants that were age-appropriately immunized were statistically less likely to be hospitalized. Of those 0-6 months of age, all 15 infants with encephalopathy and all 23 infants that died were too young to be immunized or were under-immunized. In the same age group, 95% of 68 infants with seizures were too young to be immunized or were under-immunized.

CONCLUSION:
Most cases with severe outcomes were unvaccinated or under-immunized for pertussis. Those cases that are vaccinated tend to have a milder course of disease and are not as likely to require hospitalization or to have other severe outcomes. Further exploration of the protection offered by receiving 1-2 vaccine doses is needed to direct effective interventions. Another appropriate next step would be to study the effects of vaccination in older children.

LEARNING OBJECTIVES:
To understand how vaccination status affects severity of pertussis illness.