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.