BACKGROUND:
Viral upper respiratory influenza-like illnesses (ILI) predispose to invasive pneumococcal disease (IPD). The use of heptavalent pneumococcal conjugate vaccine (PCV7) should reduce the incidence of IPD.

OBJECTIVE:
To determine if the rate of IPD is influenced by annual variation in the rate of ILI in addition to use of PCV7.

METHOD:
The number of cases per year of IPD and emergency department (ED) visits of patients from 7 hospitals in a health system were tabulated during the 2 years (7/98-6/00) prior to and 3 years (7/00-6/03) since routine use of PCV7 in young children. The incidence of ILI was based upon reports of ILI from all 13 hospitals in one county (where most of the system hospitals are located) and from reports of ILI-related absences from all schools in the same county.

RESULT:
67% of the 634 IPD cases from 1998-2003 occurred during the influenza seasons from November through April. Compared with the pre-PCV7 years, there were 67% and 72% reductions (P<0.0001) in the rates of IPD episodes (cases/10⁵ ED visits) during the PCV7 years in children <2 years and 2-4 years old, respectively. There were 36% and 28% reductions in the rates of IPD in adults 18-49 years (P<0.05) and >64 years (P<0.01) rspectively. The mean seasonal report of ILI from hospitals was 47% lower (P<0.0001) during the three PCV7 winters and the mean ILI report from schools was 18% lower (P<0.0001) than the two pre-PCV7 winters.

CONCLUSION:
In addition to efficacy of PCV7, milder ILI seasons during the PCV7 period may have contributed to an observed reduction in the IPD rates.

LEARNING OBJECTIVES:
The reader will gain knowledge that variables in addition to immunization may influence the reduction in the incidence of vaccine preventable diseases.