In this point-prevalence survey conducted in multiple states, we found that health care–associated infections affected 3.2% of hospitalized patients — a significantly lower percentage than we observed in a survey that had been conducted in 2011. These results provide evidence of national success in preventing health care–associated infections, particularly surgical-site and urinary tract infections. In contrast, there was no significant reduction in the prevalence of pneumonia or C. difficile infection, nor in the percentage of patients with health care–associated infection who died during their hospitalization, which suggests that more work is needed to prevent these infection types and reduce mortality among patients with health care–associated infections.

Although the prevalence of health care–associated infections was significantly lower in 2015 than in 2011, we did not directly compare the national burden estimates from the two surveys. Two barriers to such a comparison were present. First, there were differences in the variables that remained in the best-fitting multivariable regression models that were used in the 2011 and 2015 burden-estimation processes. For example, we lacked complete data regarding the length of stay in the hospital for patients in the 2011 survey and therefore used a proxy measure (the number of days from admission to the survey). In addition, the Nationwide Inpatient Sample underwent a redesign starting with 2012 data and was renamed the National Inpatient Sample.15

Despite differences in the methods used in the prevalence survey and in National Healthcare Safety Network surveillance, similar signals have emerged from these complementary systems, providing evidence of improvements in the safety of patients in U.S. hospitals. Analyses of National Healthcare Safety Network data through 2014, before the implementation of major changes in the definitions of health care–associated infections, showed reductions in the standardized infection ratios for central catheter–associated bloodstream infections between 2008 and 2014, selected surgical-site infections between 2008 and 2014, and MRSA bacteremia between 2011 and 2014.9 There was no reduction in the standardized infection ratio for catheter-associated urinary tract infections in hospitals nationally from 2009 to 2014, but a significant decrease in the standardized infection ratio was evident from 2013 to 2014.9

We observed significant reductions in the prevalence of urinary tract infections and surgical-site infections. Experience has shown that health care–associated infections can be prevented by means of evidence-based interventions; for example, implementation of a Comprehensive Unit-based Safety Program that was focused on catheter-associated urinary tract infection in 603 U.S. hospitals between 2011 and 2013 led to a reduction in the rates of catheter-associated urinary tract infection and urinary-catheter use.16 Reductions in urinary-catheter use, which we observed in the survey, may partially explain the lower prevalence of urinary tract infection. Although we did not collect data on urine-culturing practices, increased focus on improving the diagnosis and treatment of urinary tract infection in recent years may also have contributed.17 The reduction in the prevalence of surgical-site infections may reflect the uptake of preoperative infection-prevention practices, such as the decolonization of patients with S. aureus colonization,18-20 or the use of updated surgical prophylaxis guidelines.21 A limitation of our survey is that we do not have data to evaluate practice changes, nor do we have information about changes in the volume or types of operative procedures that may have affected the overall prevalence of surgical-site infections.

Our survey showed that pneumonia was the most common health care–associated infection, with a stable prevalence between 2011 and 2015. Similarly, an analysis of Medicare Patient Safety Monitoring System data showed that, between 2005 and 2013, the percentage of patients with ventilator-associated pneumonia among eligible Medicare patients with selected diagnoses who were undergoing mechanical ventilation remained the same, at approximately 10%.22 Although the prevention of ventilator-associated pneumonia remains an important goal, the majority of pneumonia events in hospitals in our survey were not ventilator-associated. The published literature contains relatively little regarding the prevention of non–ventilator-associated pneumonia in hospitalized patients, despite the association of this infection with poor outcomes in some reports.23,24 Some investigators have called for increased attention and resources for this underappreciated health care–associated infection.25-27

We also found that the prevalence of C. difficile infection was stable between 2011 and 2015. However, we did not collect data on changes in the use of nucleic acid amplification tests for the diagnosis of C. difficile infection in participating hospitals from 2011 to 2015. Others have suggested that increasing the use of such tests may result in an increased incidence of C. difficile infection owing to overdiagnosis.28,29 It is possible that an increased use of nucleic acid amplification tests in survey hospitals masked actual reductions in the prevalence of C. difficile infection. Analyses of National Healthcare Safety Network data have begun to show progress regarding the prevention of C. difficile infection with onset in the hospital.9 Regardless of whether changes in testing have inflated our estimate of the burden of C. difficile infection in hospitals, there is room for improvement. Because the use of antibiotics is a major driver of C. difficile infections as well as antimicrobial resistance, continued focus on improving practices for the prescribing of antibiotics is critical, in addition to infection-control measures to prevent transmission in hospitals.

Our survey has other potential limitations. As in the 2011 survey, the 2015 survey included geographically diverse sites, but the results may not be generalizable to all U.S. hospitals. Owing to the types of data available in the National Inpatient Sample, we were unable to account for all the factors associated with the prevalence of health care–associated infections in the process of developing national burden estimates. In the 2015 survey, we used the same antimicrobial screening criterion that had been used in 2011 to identify patients for review of health care–associated infections.3,30 In 2015, the proportion of patients who met the screening criterion was significantly lower than in 2011. This resulted in a lower proportion of medical records being reviewed for health care–associated infections and potentially could have resulted in the detection of fewer health care–associated infections. However, analyses of the prevalence of health care–associated infections among just those patients for whom review was performed confirmed that a smaller percentage of patients had a health care–associated infection in 2015 than in 2011, even after adjustment for other factors. Additional limitations are discussed in the Supplementary Appendix.

Prevalence surveys capture the range and relative frequencies of all health care–associated infections among hospitalized patients and complement ongoing tracking of these infections. The health care–associated infections that we identified in this survey are only one portion of the overall burden of such infections, which includes infections that occur in other settings, such as nursing homes. The CDC and the Emerging Infections Program sites are collaborating on a large-scale nursing home prevalence survey to address this gap.31 Collaborations among health care facilities, public health agencies, and other partners, bolstered by recent increases in support for programs regarding health care–associated infections, will be critical to the continued progress toward the goal of eliminating health care–associated infections.