Background: In 2009, influenza A (H1N1) infections spread worldwide. Because the use of immunomodulators is associated with an increased risk of infection, inflammatory bowel disease (IBD) patients who are on immunomodulators might be concerned about H1N1 influenza infections. The aim of this study was to investigate the age distribution and risk factors associated with H1N1 influenza of IBD patients in 2009-2010. Methods: A multicenter, prospective study was conducted, and 570 IBD patients were enrolled. Patients were followed up for 10. months to identify any new infections. The incidence and age distribution of the H1N1 influenza infections were analyzed. IBD patients with H1N1 influenza infections and 2 matched, noninfected IBD patients were selected to assess the effect of specifying the medication on the incidence of infections. Results: A total of 38 patients (6.7%) developed H1N1 influenza infections. The incidence of H1N1 influenza infections in patients aged less than 20. years was significantly higher than that among patients in other age groups (p < 0.01). The age distribution for H1N1 influenza infections in IBD patients was comparable to those in the general population. No patients needed hospitalization due to influenza infection. A total of 29 patients (76%) recovered from the H1N1 influenza symptoms within 7. days and 20 patients (53%) received antiviral treatment. The percentage of patients who used steroids or thiopurine was comparable between the cases of H1N1 influenza infection and the control group. Conclusion: Our prospective study showed that younger IBD patients were frequently infected with the influenza A (H1N1) virus as well as general population. Admission and fatal cases due to H1N1 influenza infections were not observed.
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