Decreased incidence of nosocomial invasive Methicillin-resistant Staphylococcus aureus (MRSA) infections reported through population-based surveillance in 9 metropolitan areas in the U.S., 2005 - 2006.

Background. National, regional, and local efforts to reduce nosocomial MRSA infections have begun in many U.S. hospitals over the past several years with some documented success at individual institutions. A systematic measure the larger impact of these efforts in the U.S. is lacking.

Objective. Use a national surveillance system reporting invasive MRSA to describe the change in incidence between 2005 and 2006 among nosocomial MRSA infections in comparison to non-nosocomial MRSA infections

Methods. In 9 metropolitan areas across the U.S. which include 162 hospitals participating in the Active Bacterial Core surveillance (ABCs) from January 2005 through December 2006, all persons with invasive MRSA infection (MRSA isolated from a normally sterile body site) were identified through active laboratory-based surveillance with supplemental data obtained through hospital record review. Infections were categorized into nosocomial (MRSA isolated >48 hours after admission), healthcare-associated community-onset (HACO) (MRSA isolated <48 hours since admission and any established MRSA risk factor [past hospital admission, surgery, dialysis, or long term care stay in past year, current invasive device, or any history of MRSA]), and community-associated (CA, MRSA isolated <48 hours since admission and no established MRSA risk factors). Age- and race-adjusted incidence was determined using U.S. census data and are reported as infections per 100,000 population per year. Yearly differences in age- and race-adjusted incidence were evaluated by category of infection.

Results. Overall, 9875 incident invasive MRSA infections were identified; non-mutually exclusive presenting syndromes included bacteremia (75%), pneumonia (13%), skin infection (10%), osteomyelitis (7%), and endocarditis (6%). Age- and race- adjusted incidence were similar in the two years; 31.8 per 100,000 in 2005 to 29.6 per 100,000 in 2006, a 5.8% decrease. The largest relative decrease was among nosocomial cases (9.1 vs. 8.1 per 100,000; 11.3% decrease, P<0.01), followed by HACO cases (17.9 vs. 16.6 per 100,000; 7.9% decrease, p<0.01), while there was no change in incidence of CA cases (4.8 vs. 4.6; P=0.36). Incidence of nosocomial cases decreased in 7 of the 9 reporting sites (median difference of -8.7%, range -24% to +24%).

Conclusion. Between 2005 and 2006, population-based surveillance for invasive MRSA infections in a diverse group of hospitals acrosss the U.S. demonstrated a decrease in the incidence of healthcare-associated (including nosocomial) infections but no change in CA invasive infections.

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