fragments were separated with a Chef DR III (Bio-Rad Labo- ratories, Hercules, CA) for 24 hours with initial and final switching times of 10 seconds and 60 seconds, respectively. PFGE profiles differing by fewer than four bands were inter- preted as identical or closely related (16). Bionumerics soft- ware, with Dice coefficient and unweighted pair group matching average for clustering, and optimization and toler- ance values of 1% and 1%–2%, respectively, were used to ver- ify the relatedness of PFGE profiles. MRSA isolates sharing identical or closely related PFGE profiles and phage types were considered to be the same strain type. If the PFGE pro- files were related, but the phage types were different, ribotyp- ing with one to three restriction enzymes (HindIII, EcoRI, ClaI) was performed to verify the relatedness of isolates (11). Riboprofiles with fewer than four bands’ difference were regarded as identical or closely related. The genomic variation within the mec determinant hypervariable region (HVR) was analyzed for a subset of MRSA isolates as described (12). In brief, the two probes which were prepared from plasmid pBBB30 (17) recognize the mec hypervariable region. The probes are hybridized with genomic bacterial DNA which is first digested with EcoRI and BglII. Study Population, MRSA Isolates, and Definitions All MRSA-positive persons in Finland from 1997 to 1999 and their MRSA isolates (one from each) were included in the study. A sporadic strain was defined as a strain type isolated from one person only. An MRSA isolate was defined as hospi- tal acquired if the MRSA-positive specimen was obtained 2 days after hospital admission, and for discharged patients, if the patient had been hospitalized within 2 years before the date of MRSA isolation. Previous Contacts with a Health-Care Facility National identity codes for each person with an MRSA- positive culture were obtained either through the primary diag- nostic laboratory or from the infection control nurse of the health-care facility. Based on the first isolation date of MRSA and the national identity code, data on previous hospitaliza- tions within 2 years before the MRSA isolation date were retrieved from the National Hospital Discharge Register (HILMO). The HILMO is a civil register comprising compre- hensive health-care records, provided by all hospitals and health-care centers in Finland, including outpatient surgery. Each report to the register includes patient identity informa- tion, admission and discharge dates, a code of the health-care provider, type of service, specialty, the place (home or institu- tion) from which the patient came to the institution, and data on surgical procedures. Additional background information was collected for per- sons for whom no HILMO reports could be found by sending questionnaires to infection-control nurses at relevant health- care facilities. The information collected included 1) whether the MRSA-positive person was a patient or a staff member, 2) whether the specimen was taken on clinical or screening basis, RESEARCH and 3) whether the screening sample was taken because of a hospital contact abroad or because of an epidemic situation. Statistical Analysis and Ethical Aspects For categorical variables, proportions were compared by the chi-square test with Yates correction or Fisher’s exact test, as appropriate. The means and medians of the continuous vari- ables were compared by the Student’s t test or Mann-Whitney U test, depending on the sample distribution. With approval from the Ministry of Social Affairs and Health and the Finnish data protection authority, the National Research and Development Centre for Welfare and Health gave permission to use the data from the HILMO register. Results From 1997 to 1999, 520 MRSA isolations were reported to the National Infectious Disease Register; the annual incidence ranged from 2.3 to 4.1/100,000 persons. The Laboratory of Hospital Bacteriology received MRSA isolates from 529 per- sons. Three persons did not have a Finnish national identity code and were excluded from the study. The median age of the 526 persons was 51 years (range <1–96), and 291 (55%) were male. Contacts with Health-Care Facilities Of the 526 MRSA-positive persons, 108 (21%) did not have any verified link to health-care facilities 2 years before the MRSA isolation date, including 17 persons whose MRSA was isolated within 2 days of hospital admission. Their MRSA isolates were classified as community acquired. BEcause of signs of infection or clinical findings, specimens were taken from 69 persons and, 37 persons were specifically screened for MRSA. Of those 37, 21 had a known MRSA contact in their family, 13 were otherwise exposed to a known MRSA carrier, and 3 were known to have carried MRSA previously. The HILMO register and the questionnaire survey showed 418 (79%) persons who had at least one connection to a hospi- tal, and their MRSA isolates were classified as hospital acquired. According to the HILMO register, 376 persons were hospitalized in Finnish hospitals within 2 years before the MRSA isolation date: 156 hospitalized patients with MRSA isolated 2 days after hospital admission and 220 discharged patients. The time frame between the MRSA isolation date and the previous hospitalization for the discharged patients was <6 months for 156 (71%) patients, 6 to 12 months for 36 (16%), and >12 months for 28 (13%) patients. The questionnaire sur- vey identified 42 additional persons who had an obvious con- tact with a hospital, including 23 staff members (12 working in a Finnish health-care facility and 11 who had recently worked abroad) and 19 patients, 15 of whom had recently been hospi- talized outside Finland. The median age of the persons who did not have a contact with a health-care facility was lower than that of persons who had contact (34 vs. 58 years, p<0.01). The proportion of chil- dren <15 years of age was higher in community-acquired than Emerging Infectious Diseases • Vol. 8, No. 6, June 2002 603