In the present study, the hospitalization rate, rate of pneumonia, ICU admission rate, and case fatality rate among patients with confirmed symptomatic pandemic influenza A (H1N1) were 6%, 3.6%, 0.7%, and 0.1%, respectively.
Rate of hospital/ICU admission and pneumonia in male were about 2 times higher than female especially in the more than 50 years-old pandemic influenza A(H1N1) 2009 confirmed cases. There was also higher rate of ICU admission in male, while statistics did not reach to meaningful. Different behavior, hormone responses, and susceptibility to infectious disease may lead to distinct profiles of pandemic influenza A(H1N1) 2009-associated morbidity for male and female.
The severity of infection may be measured in a variety of ways; the case-fatality rate, which is the probability that an infection causes death, is the simplest method by which to measure the severity of infection. The case-fatality rate may be influenced by the denominator (the non-fatal patients). Asymptomatic H1N1 virus infections or patients with mild disease were often thought to have a simple cold, and therefore overlooked (undiagnosed). Therefore, diagnostic strategies of the H1N1 virus may have a big influence on the overall fatality rate. The severity of infection may be measured based on the hospitalization cases and ICU cases (the probabilities that an infection leads to hospitalization or ICU admission). The age-specific severity patterns, as estimated herein, are different from the age-specific severity patterns which would be obtained by simply comparing the confirmed cases, hospitalizations, and deaths in the US as a whole for a similar period of time [7, 14].
The estimation of the case fatality rate using the laboratory-confirmed cases in the denominator resulted in a rate of 0.82%. This finding is consistent with other studies that have used laboratory-confirmed cases as a denominator [15, 16]. The early epidemiologic features of mortality from the pandemic H1N1 infection were different in relation to the region, and the majority of H1N1 virus infection-related deaths occurred in patients 20-49 years of age . The calculated case-fatality rate in Asian countries is lower than on the other continents . The case-fatality rate may have been influenced by the denominator (the non-fatal patients). The mortality rate for symptomatic illness was estimated to be 0.048% in the United States  and 0.026% in the United Kingdom .
In the present study, the denominator was confirmed symptomatic patients with H1N1; the overall mortality was 0.1%, which was comparable to previous studies . However, the mortality of elderly patients infected with pandemic H1N1 is higher in Asian countries . The elderly appear to be less infected from pandemic influenza A (H1N1), which may be due to a lack of exposure. However, when infected, the elderly are more likely to have a fatal outcome than younger patients .
The first pandemic influenza A (H1N1) virus-infected patient in the US was confirmed on 15 April 2009 . Although the first H1N1 patient was confirmed in May 2009, a surge in the number of infected patients was observed 5 months later (October 2009) in KUDH (Figure 1). National H1N1 influenza vaccination program in Korea has started for medical personnel on October 27, and extended in general population on November 1. At the same time with vaccination program has started, H1N1 confirmed cases were declined.
We can confirm all of the H1N1 cases by RT-PCR without delay, even on holidays, and suspected H1N1 cases are treated with oseltamivir before the RT-PCR results are available. These efforts could change the epidemiologic features. The mean time from hospital admission to initiation of antiviral treatment was 1.5 days in the present study, which was more rapid than a previous study .