[A case-control study of an influenza A (H1N1) outbreak in a hospital].

OBJECTIVE This study aimed to explore the epidemiological factors of an influenza A (H1N1) outbreak in a hospital. METHODS General data were collected via face-to-face interview and telephone survey. Total 132 individuals including medical and nursing staffs (37), in-patients (39) and patients' family members (56) who were exposed to the pediatric surgery ward during August 11 - 18, 2009, were investigated. The case group included 35 cases according to the diagnostic criteria for influenza A (H1N1). The other 97 persons were grouped as control. A case-control study was then conducted to explore the epidemic factors, and layering analysis was applied to determine the interactions among these factors. RESULTS The overall incidence in this study was 26.5% (35/132), which included 12 confirmed and 23 suspected cases, and there was no severe case. The first case was a child with the influenza-like symptoms before admission on August 11. The onsets of these cases were during August 7 - 17. The cases were distributed in 9 of 13 rooms, and there was no room aggregation in the cases distribution (χ(2) = 0.00, P > 0.05). Twelve of 25 oropharyngeal swabs were influenza A (H1N1) nucleic acid positive. The case-control study showed that exposure to the enema room accounted for 93.10% (27/29) in cases and 72.73% (48/66) in control; OR = 5.06, 95%CI = 1.01 - 34.23), long time exposure to ward was 71.43% (25/35) in cases and 44.33% (43/97) in control; OR = 3.14, 95%CI = 1.27 - 7.90), and short distance contact with the nurse LIU (76.46% (26/34) in cases and 50.52% (49/97) in control; OR = 3.18, 95%CI = 1.22 - 8.54) were the risk factors. However, keeping the window open (27.59% (8/29) in cases and 68.18% (45/66) in control; OR = 0.14, 95%CI = 0.05 - 0.39) and hand washing (25.71% (9/35) in cases and 76.29% (74/97) in control; OR = 0.11, 95%CI = 0.04 - 0.28) were the protective factors. The longer time exposure to ward had the higher risk (ratios of cases to control were 4:20 (0 - 1 day), 6:34 (2 - 4 days) and 25:43 (≥ 5 days); χ(2)(trend) = 5.737, P < 0.05). In contrast, hand washing with more frequencies (ratios of cases to control were 26:23 (0 - 1 time one day), 7:9 (2 - 3 times one day) and 2:65 (≥ 4 times one day); χ(2)(trend) = 37.136, P < 0.01) and the longer time window opening (ratios of cases to control were 21:21 (no), 4:13 (a few) and 4:32 (often); χ(2)(trend) = 13.830, P < 0.01) had the lower risk. Nevertheless, layering analysis excluded long time exposure to ward from the risk factors (for individuals with more frequent hand washing, 6.90% (2/29) exposed in cases, 7.14% (1/14) exposed in control, OR = 0.97, 95%CI = 0.06 - 29.51; for individuals keeping window open, 21.21% (7/33) exposed in cases, 8.33% (1/12) exposed in control, OR = 2.55, 95%CI = 0.26 - 60.87), indicating the main risk factors in this outbreak were exposure to the enema room and short distance contagion with the infected nurse. CONCLUSION The influenza A (H1N1) outbreak in this hospital was induced by an inpatient infected with influenza A (H1N1) virus before admission. Infected medical staffs keeping on work and exposure to the same place, e.g.the enema room in this study might spread the influenza A (H1N1) virus, and frequent hand washing and keeping the window open are the most effective and economic methods to prevent influenza A (H1N1) infection.