RESEARCH in hospital-acquired MRSA strains (27 [25%] of 108 vs. 31 [7%] of 418, p<0.01). Strain Types Among the 526 MRSA isolates, our typing scheme showed 84 strain types, 56 (67%) of which were sporadic and 28 (33%) shared by at least two persons. The distribution of sporadic (in total 56 [11%] of 526) and shared (in total 470 [89%] of 526) strain types was similar in persons with and without connections to health-care facilities (Table 1). Fourteen strain types, each of which were isolated from >10 persons, represented 421 (80%) of 526 MRSA isolations (Table 2, Figure). Three of the 14 most common strain types were more likely to be found in persons who did not have a contact with a health-care facility than in those who had such a contact: Mikkeli clone (41 [38%] of 108 vs. 75 [18%] of 418, p<0.01), E31 (10 [9%] of 108 vs. 6 [1%] of 418, p<0.01), and E22 (8 [7%] of 108 vs. 7 [2%] of 418, p<0.01). Of all strains isolated from persons who had no hospital contact, 94% were nonmultiresistant. In addition, of the 14 most common strain types, all 7 nonmultiresistant strains, but none of the multiresistant strains, showed HVR type A (Table 2). Of the 56 sporadic MRSA strains, 41% were nonmultiresis- tant, including all but one of the strains isolated from persons without connections to hospitals (Table 1). Discussion Our population-based study showed that from 1997 to 1999 one fifth of all Finnish MRSA isolates came from per- sons who had no connection to health-care facilities, suggest- ing that these MRSA isolates may be community acquired. Three strain types identified by phage typing, PFGE, and ribotyping were associated with community acquisition, and none of these strain types were multiresistant. To our knowledge, this is the first report of community- acquired MRSA on the national level. Previous reports have focused on single health-care institutions or certain restricted areas (1,3,6,12,18-20). Both data sources used in our study, the surveillance and typing scheme of MRSA and the hospital dis- charge register, were nationwide (20). The availability of national identity codes allowed us to link the two data sources and to study one MRSA isolate per person. The number of iso- lates routinely typed and verified as MRSA was equal to that of MRSA isolations reported to the National Infectious Dis- ease Register, suggesting that the isolates of MRSA available for typing were nationally representative. Community-acquired MRSA can be classified into the fol- lowing categories: discharged hospital patients with MRSA, nursing-home residents with MRSA, MRSA transmitted to nonhospitalized patients, and MRSA arising de novo in the community (7). The first three categories include MRSA iso- lates of health-care facility origin, which are thought to repre- sent a limited number of different genotypes, disseminate clonally, and express resistance to multiple antibiotics (1,21– 23). De novo MRSA strains, in contrast, are thought to arise Table 1. Distribution of sporadic strains of methicillin-resistant Staphylococcus aureus (MRSA) and strains shared by at least two persons and contact with a health-care facility, Finland Persons with hospital contact (%) Persons without hospital contact (%) n=418 43 (10) 375 (90) n=108 13 (12) 95 (88) Sporadic strain types Strain types shared by at least two persons through acquisition of mec DNA into a previously susceptible S. aureus genotype (7,9,24). Our study focused on the last two categories. We first identified nonhospitalized MRSA-positive persons and thereafter compared their strains with those found in hospitalized patients. Based on our study, the proportion (21%) of community- acquired MRSA was relatively high in Finland. However, the definition of community-acquired MRSA is not straightfor- ward. The definition classically includes MRSA isolated out- side hospital settings or from a patient within 48 to 72 hours of hospital admission. Because of the long-term persistence of MRSA colonization (25), contacts with hospitals or nursing homes before MRSA isolation should also be taken into account (6,19,26). Therefore, our definition of community acquisition covered a 2-year time period without a health-care facility contact before the MRSA isolation. If the cut-off period had been 1 year, the proportion of community-acquired isolates would have been 26%. If the questionnaire survey on the additional background data had not been performed, the proportion would have been 29%. Although this questionnaire survey was not comprehensive, it allowed us to characterize the persons who had no reports in the discharge register and to identify persons with a foreign or ongoing hospital contact. Most community-acquired MRSA strains were nonmulti- resistant, and children were more likely to have a community- than a hospital-acquired MRSA. These findings agree with those of previous reports, suggesting that nonmultiresistant MRSA is emerging as an important pathogen in the commu- nity (1,18,22,23). The majority (64%) of community-acquired strains were isolated on a clinical basis. However, one third of all community-acquired strains were isolated because of screening of persons exposed to a known MRSA carrier, most of whom were family members. Most exposed persons had the same strain type as their contacts (data not shown). Among the 14 most common strain types, which represent 80% of all MRSA isolates, we identified 7 nonmultiresistant strain types showing a hypervariable region hybridization pat- tern A. Three of these strain types were associated with com- munity acquisition and represented more than half of all community-acquired strains. In addition, three other strain types were frequently found in persons without connections to hospitals. The only exception was the internationally recog- nized UK EMRSA-15 (27), which was isolated mainly from patients or health-care workers who had recently returned from hospitals abroad. In contrast, the multiresistant strain types, including the UK EMRSA-16 (28) and the Iberian clone 604 Emerging Infectious Diseases • Vol. 8, No. 6, June 2002