Learning Objectives for this Presentation:
By the end of the presentation participants will be able to identify determinants influencing the epidemiology of pertussis.

Background:
Pertussis is a unique VPD whose morbidity hasn't been completely curtailed by immunization. Pre-vaccine era morbidity peaks, every 2-5 years, remain consistent and poorly understood. In Los Angeles County (LAC), incidence rose to 4.6/100,000 persons in 2005. Peaks in incidence also occurred in 1999 and 2002.

Objectives:
Examine factors that are influencing the epidemiology of pertussis.

Methods:
Reported LAC cases' demographics and symptom onset dates were analyzed in the year preceding, during, and following the recent surge in incidence (2005) and in all recent peak incidence years (1999, 2002, 2005). Age categories analyzed were infants(<6 months), children(6 months-9 years), adolescents(10-19 years), and adults(20+ years).

Results:
Cases among adolescents/adults have risen since 2003, with infants now comprising <50% of cases. During the winter preceding all peak incidence years, the proportion of adolescent cases increased at least 95% from the previous winter and remained at >22% through 2005 and into winter/spring of 2006. From 1999 to 2002, the percent of adult cases that were White increased from 41% to 50%. During 2004-2006, Whites accounted for >55% of annual adult cases. In 2002, 29.4% of adolescents were White, compared to 57.9% in 1999 and >54% during 2004-2006. Hispanics comprised >50% of infant/children cases in all peak incidence years and 2004-2006. Geographic locales with highest overall 1999/2002 annual morbidity were not the highest in 2004-2006: Five locales contributed 38.5-58.1% of adolescent/adult cases during all peak incidence years and 2004-2006. A 2:1 female to male ratio was observed during all peak incidence years and 2006.

Conclusions:
Age, gender, seasonal, racial/ethnic, and geographic determinants are all equally shifting the LAC pertussis epidemiology, contributing to the consistency of peak incidence years. Routine assessments of LAC pertussis epidemiology are required to ensure effective outreaches/interventions.