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We read with interest the published article by Poorolajal et al in your journal recently. All reports from Iran confirmed that the epidemiology of hepatitis B virus (HBV) infection has been decreasing during recent years to about 2% of the general population. The infection is less in blood donors and more in intravenous drug users (IDUs). Hepatitis C is an emerging disease in our community due to IDUs. Poorolajal et al found that the prevalence of hepatitis C virus (HCV) infection was increasing, and we agree with. Authors have mentioned that they had excluded those cases whose infections were already diagnosed in previous years and those who lived in other provinces. Since their data collection was based on the data registered in Hamadan Province Health Center database, and there were no such data in these centers before 2004, it is difficult to differentiate the cases detected in these years from the ones detected before 2004 that were not registered. It is important to know how they found or gathered such data. Moreover, we have temporary immigrations in our country that can affect the epidemiology of HBV infection. HBV and HCV infected cases are usually chronic patients without symptoms and may have been infected already. They are usually diagnosed accidentally during blood donation, screening during pregnancy, due to a positive case in the family, for being hired and so on. Therefore, we are not sure that the presented data in results, tables, and figures show the incidence rate. What about the at-risk-population (the denominator of the incidence rate) if these are incidence rates? It seems that these are prevalence rates. Based on the cause of detection of the infection, these cases could be found eventually and these rates would be changed greatly. Therefore, these rates should be discussed more conservatively. It seems that it is a cross-sectional study but not a retrospective cohort. Actually, they did not follow a group of cases to estimate the incidence rate, so they cannot conclude the results of the study as a cohort. Variability of past exposures across successive generations (birth cohorts) can distort the apparent association between age and these prevalence rates. Age effect and cohort effect are mixed with the results. Therefore, we cannot necessarily conclude that there is a decreasing trend in the incidence rate of HBV infection or fluctuating trend about HCV in this study setting. Authors have shown that the distribution of hepatitis B and hepatitis C infections are highest in the third and fourth decades of life, respectively. Since data are being age-adjusted, these findings can be related to the age distribution of the residence of Hamadan Province. However, cause of detection is important as

Comment on: Epidemiology of Hepatitis B Virus Infection in Hamadan, West of Iran.