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We sought to describe the epidemiology of pneumonia hospitalizations in Minnesota over 12 years, before and after the introduction of conjugate vaccines.
Patients and Methods
We conducted a retrospective study of pneumonia hospitalizations from 1999 through 2010 based on hospital discharge codes in the Fairview Health Services network in Minnesota, a state with 96.7% uptake of PCV7 among children. Fairview Health Services in Minnesota encompasses 6 hospitals in the greater Twin Cities (Minneapolis and Saint Paul) of Hennepin County, serving more than 1 million patients with 175,000 emergency department visits and over 73,000 inpatient admissions per year. A case of pneumonia hospitalization was defined as a record with any of the following International Classification of Diseases, Ninth Revision (ICD-9) codes assigned as the primary diagnosis: 073.0, 481, 482.2, 482.31, 482.32, 482.39, 482.40, 482.41, 482.42, 482.49, 482.89, 482.9, 483.0, 483.1, and 486. A primary diagnosis of empyema was identified using ICD-9 codes 510, 510.0, and 510.9.
Cases were grouped by age (<2 years, 2-4 years, 5-18 years, 19-49 years, 50-64 years, and 65 years and older) and ICD-9 code. The study dates span the pre-PCV7 (1999-2000), early post-PCV7 (2001-2004), and late post-PCV7 (2005-2010) eras. Standard linear regression was used to calculate trends for 1999-2004 and 2005-2010, and a formal test of the interaction was completed to determine if the trends differed between time periods.
A total of 14,807 hospitalizations were documented with a pneumonia-related ICD-9 diagnosis between 1999 and 2010. From 1999 through 2004, the pre- to early post-PCV7 periods, a significant decrease in pneumonia hospitalizations was seen for children less than 2 years of age (P=.028) and those in the 2- to 4-year age group (P=.032), as well as among adults in the 50- to 64-year age group (P=.039) and among those over 65 years (P=.008). A significant decrease in pneumonia hospitalizations was also observed from 2005 through 2010 for children less than 2 years of age (P=.013) and children between the ages of 2 and 4 (P=.033), but no significant trend was observed for older individuals. The most common pathogen-related pneumonia diagnosis was pneumococcal pneumonia, followed by staphylococcal pneumonia (Figure; data was based on ICD-9 codes, and staphylococcal pneumonia represents all relevant ICD-9 codes [482.40, 482.49, 482.41]. As stated in the limitations, the largest diagnostic entity was the general diagnosis of ‘pneumonia’ but we did not abstract the data to ascertain the pathogens involved).
Our findings indicate that pneumonia hospitalizations decreased in both young children and older adults in the early post-PCV7 period, although the decrease was only sustained in the younger children. This finding has been noted in other studies.
In contrast, the availability of the 23-valent pneumococcal polysaccharide vaccine, which has been effective in reducing pneumococcal bacteremia, has not reduced the risk of community-acquired pneumonia.
An increase in methicillin-resistant Staphylococcus aureus pneumonia in the late post-PCV7 period may be a result of nasopharyngeal eradication of vaccine-related Streptococcus pneumoniae serotypes and replacement by S aureus. However, while all staphylococcal pneumonia diagnoses significantly increased from the pre-PCV7 era to the early post-PCV7 era (P=.049), a nonsignificant decrease was observed in the late post-PCV7 era (P=.10), which includes the 2009-2010 influenza pandemic that was associated with an increase in secondary bacterial pneumonia.
but no statistically significant trend was evident in our data.
One must be wary of drawing too specific a conclusion from our findings because the trends were based solely on ICD-9 codes, which can be inaccurately recorded by clinicians. Furthermore, most of the ICD-9 code diagnoses were pneumonia and bacterial pneumonia and were not pathogen specific; hence, we could have potentially increased the number of confirmed pathogens if we did an in-depth chart review. We did not abstract the clinical, radiologic, and microbiological data to confirm the diagnosis. Statistically, we did not perform multiple comparison adjustment. Appropriately controlling for multiple comparisons is difficult in this case due to the correlation between end points, but none of the trends would have been significant with a conservative Bonferroni adjustment.
This study provides preliminary findings that indicate, in a state where a high percentage of children are vaccinated with PCV7, the possible impact of childhood immunization with pneumococcal conjugate vaccination on pneumonia hospitalizations of adults and children and a possible correlation with staphylococcal pneumonia hospitalizations.
We thank Heather Jerry for her assistance with statistical analysis.
Active Bacterial Core Surveillance Team
Changing epidemiology of invasive pneumococcal disease among older adults in the era of pediatric pneumococcal conjugate vaccine.