Trends in invasive pneumococcal disease among older adults in Olmsted County, Minnesota

Following the introduction of a 7-valent pneumococcal conjugate vaccine (PCV-7) for children in early 2000 in the United States, a decrease in the incidence of invasive pneumococcal disease (IPD) was seen in adults, likely due to a herd effect. However, there have been recent increases in IPD in adults caused by Streptococcus pneumoniae serotypes not included in PCV-7, so called "replacement disease". We performed a population-based study to further investigate this emerging concern.

Population-based incidence study in Olmsted County, Minnesota, United States, in adults aged > or =50 years.

From 1/1/1995 to 12/31/2007, 104 cases of IPD were identified in Olmsted County in adults aged > or =50 years. We found a 45% increase in the incidence rate of IPD from 2001-2003 (17.7 cases per 100,000 person-years) to 2004-2007 (32.1 cases per 100,000 person-years) (p=0.029). From 2002-2004 to 2005-2007, the incidence rate of IPD caused by S. pneumoniae serotypes not included in PCV-7 increased from 9.2 to 32.8 cases per 100,000 person-years (p<0.001).

A recent increase in the incidence of IPD in adults aged > or =50 years was demonstrated in Olmsted County, Minnesota due to serotypes not found in PCV-7. These findings are unique and merit further investigation.