Impact of the 2009 influenza pandemic on pneumococcal pneumonia hospitalizations in the United States

Abstract

Background: Infection with influenza virus increases the risk for developing pneumococcal disease. The A/H1N1 influenza pandemic in autumn 2009 provided a unique opportunity to evaluate this relationship.

Methods: Using weekly age-, state-, and cause-specific hospitalizations from the US State Inpatient Databases of the Healthcare Cost and Utilization Project 2003-2009, we quantified the increase in pneumococcal pneumonia hospitalization rates above a seasonal baseline during the pandemic period.

Results: We found a significant increase in pneumococcal hospitalizations from late August to mid-December 2009, which corresponded to the timing of highest pandemic influenza activity. Individuals aged 5-19 years, who have a low baseline level of pneumococcal disease, experienced the largest relative increase in pneumococcal hospitalizations (ratio, 1.6 [95% confidence interval {CI}, 1.4-1.7]), whereas the largest absolute increase was observed among individuals aged 40-64 years. In contrast, there was no excess disease in the elderly. Geographical variation in the timing of excess pneumococcal hospitalizations matched geographical patterns for the fall pandemic influenza wave.

Conclusions: The 2009 influenza pandemic had a significant impact on the rate of pneumococcal pneumonia hospitalizations, with the magnitude of this effect varying between age groups and states, mirroring observed variations in influenza activity.

Figure 1.

Weekly pneumococcal pneumonia hospitalizations per 100 000 in 2009 (black solid line) compared with 2003–2008 baseline 95% prediction interval (gray solid and dashed lines) for age groups (A) 0–4, (B) 5–19, (C) 20–39, (D) 40–64, and (E) ≥65 years. Weeks 15–50 (April–December) of the 2009 calendar year are shown, with the baseline calculated from the simple empirical approach. The shaded circles on the bottom of each panel indicate the number of influenza hospitalization in each week across all states and age groups, with darker shades indicating more hospitalizations.

Figure 2.

Timing of influenza compared with the timing of excess pneumococcal pneumonia in fall of 2009 among 5–64-year-olds. Timing is defined as the average of the week number (34–50 of 2009) weighted by the number of influenza hospitalizations or excess pneumococcal pneumonia hospitalizations in that week. Excess pneumococcal pneumonia hospitalizations for each state were calculated using the simple empirical baseline method. R² = 0.43 from a linear regression weighted by population size of each state (P < .001).