KEYWORD1:
Pertussis, Epidemiology

BACKGROUND:
June-September is traditionally the season of highest pertussis incidence in Los Angeles County (LAC) and California. A high number of cases occurred during October-January of 2001-2002 in LAC.

OBJECTIVE:
To reassess the epidemiology of pertussis in LAC.

METHOD:
Probable/confirmed cases with disease onset in February 1991-June 2002 were organized by seasonal group: February–May, June-September, October-January. A stratified analysis was performed on three categories: 1991-2001, 1996-2001, 2002. October-January 2001-2002 cases were compared to a similar number reported during the same period in 1999-2000 and 1992-1993.

RESULT:
During January-June 2002, 66 cases were identified, a 43% increase from the same period yearly average in 1996-2001. This rise is attributable to the number of January 2002 cases (n=18). There were 55 cases reported with onset in October-January, 2001-2002, a 77.4% (n=31) increase from the yearly average of the same periods in both 1991-2001 and 1996-2001. In October-January 1992-1993 and 1999-2000, 22% and 25% of reported cases, respectively, were epidemiologically linked to each other. In 2001-2002, only 6% (n=3) of cases were linked. Infants < 1 year comprised the majority of cases in all seasonal groups since 1991. However, children 5-14 years accounted for 20% (n=11) of October-January 2001-2002 cases, up from a 7.2% (n=3) and 9.7% (n=3) average yearly contribution during the same period in 1991-2001 and February-May 1991-2001, respectively. From February-May 2002, this age group comprised 5% (n=2) of cases. Of the 37 cases in children 5-14 years reported during October-January 1991-2002, 11 (30%) occurred during 2001-2002. Detection and reporting biases were analyzed and ruled out.

CONCLUSION:
With the winter peak in 2001-2002, LAC will experience an increased number of cases in calendar-year 2002. Cases reported during 2002-2003 will be monitored. Active surveillance efforts may need to be focused on 5-14 age group.
LEARNINGOBJECTIVES:
To underscore the importance of evaluating the seasonal epidemiology of pertussis in a community.