influenza a (h1n1) in hamedan province, western iran in 2009: a case-control study

background: the novel influenza a (h1n1) virus was first detected in march 2009 in mexico and then disseminated to many other countries worldwide. in this study, we assessed the potential risk factors of swine flu as well as the most important clinical manifestations of this infectious disease among confirmed cases during early phase of pandemic h1n1. methods: subjects (cases and controls) were selected from those patients with signs and symptoms of respiratory tract infection who referred to health centers of eight cities throughout hamedan province, western iran form july to december 2009. characteristics of the participants were obtained by interviewers using pre-determined questionnaire. cases were distinguished by pharyngeal soap speciâ­mens positive for influenza a virus using polymerase chain reaction (pcr). logistic regression model was conducted at 0.05 significance level using stata 9.1 statistical software to assess the effects of various risk factors on h1n1 influenza infection. results: totally, 245 confirmed cases of h1n1 influenza were compared with 388 controls. case fatality rate of influenza infection was about 2.86%. in comparison with age group of 1-19 yr old, adjusted odds ratio estimates was 1.91 [95% ci: 1.06, 3.46] for age group of 20-39 yr old, 0.94 [0.37, 2.38] for age group of 40-59 yr old, and 0.34 [0.09, 1.37] for age group of 60-79 yr old. adjusted odds ratio estimates of influenza a infection was 8.12 [95% ci: 3.11, 21.6] for pregnant women compared to non-pregnant women; 1.84 [95% ci: 1.32, 2.86] for high educated individuals in comparison with low educated individuals; 2.11 [95% ci: 1.25, 3.57] for whose who had close contact with suspected influenza patients; and 2.15 [95% ci: 1.16, 3.98] for individuals with normal body mass index (bmi= 25-30) compared with underweight individuals (bmi there were no significant differences in clinical manifestations between cases and controls. conclusion: the risk of influenza a infection is highest among children and adolescents, pregnant women, high educated individuals, and those who had close contact with suspected influenza patients during pandemic phase. in addition, there is no pathogonomonic sign or symptom to distinguish influenza infection clinically from other kinds of respiratory track infections.