In this study, we observed differences in the age distribution and risk factors for severe illness between the first winter season of post-pandemic period and the 2009–2010 pandemic period. A shift to older ages in the age distribution of hospitalized and fatal patients were observed during the winter season of 2010–2011, which was consistent with data from the United Kingdom, Greece and Taiwan 
. During the winter season of 2010–2011, children aged 0–4 years and adults aged 65 years or older had the highest risk ratios of hospitalization, while people under 25 years of age had the highest risks of hospitalization (peak 5–14 years) during the 2009–2010 pandemic. During the winter season of 2010–2011, risk ratios of hospitalization in the 5–14 and 15–24 years age groups declined, compared with the 0–4 years age group. The change of higher risk age groups might be explained by highest immunity to 2009 H1N1 in the 5–14 and 15–24 years age groups after experiencing the pandemic wave which was reported from serological study in China and other countries 
. The high risk of death due to 2009 H1N1 were consistently observed among children of 0–4 years and older adults aged 65 years or older during the winter season of 2010–2011 and the 2009–2010 pandemic. For children aged 0–4 years, the greater risk for hospitalization than for death with 2009 H1N1 infection may have resulted from a lower threshold for hospital admission and therefore inflate the calculated Risk Ratio compared to other age groups. During the 2009–2010 pandemic, studies in several countries reported that obesity was associated with severe or fatal 2009 H1N1 virus disease 
. Although our study indicated the proportion of obesity among hospitalized patients was significantly higher than the general Chinese population, obesity among hospitalized cases was not a statistically significant risk factor for severe complications from 2009 H1N1 virus infection during the winter season of 2010–2011. This is in contrast to a previously published study in China during the 2009–2010 pandemic 
. The absence of an association between obesity and severe outcomes may be explained by the higher proportion (40.8%) of chronic medical conditions among obese patients who were admitted hospitals in our study, compared to the previously published study in China (24%). Additionally, the number of obese patients in this study was small limiting statistical power to detect an association with severe outcomes.
Consistent with studies describing seasonal influenza and other studies about the 2009 H1N1 pandemic 
, the presence of at least one chronic medical condition was associated with 2009 H1N1 severe illness. In our study, a higher proportion of severe cases had at least one underlying medical condition (47.4%) was observed compared to the previous study conducted during the pandemic period in China (33%) 
Consistent with the previous studies of seasonal influenza and 2009 H1N1 pandemic, our results reaffirmed that early initiation of oseltamivir treatment may reduce the risk of influenza-associated complications. However, our study observed lower usage of antiviral therapy (55.9%), compared to the previously published study from the pandemic period in China (76%) 
. The proportion of antiviral treatment within 2 days from symptom onset in our study was low (26.0%), but higher than the study of hospitalized cases (17%) in China during the pandemic period 
. Some reasons for the delay in treatment initiation included waiting for laboratory confirmation of 2009 H1N1, delays in healthcare presentation, or the reduced awareness of antiviral treatment. Although antiviral treatment is accessible at different healthcare settings, our study showed only a small proportion of patients received antiviral treatment before admission to the hospital. According to current Chinese influenza surveillance data, nearly all 2009 H1N1, H3N2 and B virus strains tested were susceptible to neuraminidase inhibitors (oseltamivir and zanamivir) 
. People with underlying medical conditions and other possible risk factors for severe disease from influenza virus infection should be educated to seek treatment promptly after onset of an influenza-like illness to ensure that antiviral treatment if appropriate is initiated in a timely fashion. Recommendations to healthcare providers should suggest providing early empiric treatment with appropriate influenza antiviral medications to suspected cases of influenza virus infection, both in outpatient settings and inpatient wards, especially to those patients who may be at higher risk of influenza virus infection complications.
Our findings indicated that male patients were more likely to develop severe illness, which was consistent with the previously published study in China during the 2009–2010 pandemic period 
. Nevertheless, a global pooled analysis showed that men were approximately half of all hospitalized, ICU-admitted, and fatal cases10.
It was also observed in studies from South Korea, that mean had a significantly higher proportion of pneumonia 
. The association between men and severe illness of 2009 H1N1 may reflect different behaviors, underlying medical conditions, susceptibility to 2009 H1N1 virus infection and other unrecognized risk factors for severe illness among men.
Our study had a number of limitations that should be noted. The reported hospitalized patients in this study only represented a portion of the total number of actual hospitalized patients with 2009 H1N1 infection due to limitations of the clinical surveillance system in capturing individuals who seek medical care at hospitals and obtain laboratory test. There is a decrease of the numbers of influenza-confirmed patients and hospitalized patients during the winter of 2010–2011 compared to 2009–2010 pandemic period. This decline may due to more under-reporting (compared to a more strengthened surveillance during pandemic period) or due to a high immunity level against 2009 H1N1 in the population. Some of the associations with age groups may have been due to underreporting or overreporting of cases in any one group. Chart abstractions or submission of medical records to China CDC were performed voluntarily, rather than systematically which reflects the willingness and capacity of physicians to perform them. In this case series, the high death to hospitalization ratio (7.8%) may be a result of case referral bias in this voluntary case review/submission process. Our study may be biased towards older adults in the analysis of risk factors because patients who had a chart review were older, compared with those patients without chart review. Influenza vaccine information of many hospitalized cases were missing in this study because vaccine history is not a required data in medical records in most of hospitals in China. Thus, our findings should be interpreted with caution because of the retrospective study design, selection bias and small sample size.
Despite our study limitations, we observed some important trends in severe infection with 2009 H1N1 virus infection. This study indicated age groups at higher risk of hospitalization during the immediate post-pandemic period were changing, compared with those during the 2009–2010 pandemic. During the winter season of 2010–2011, children under 5 years had the highest risk of hospitalization and death associated with 2009 H1N1 infection. Additionally, a decline of risk of hospitalization among people aged 5–24 years and a shift to older age for fatal patients was detected. The relative risk of hospitalization and death among people older than 64 years increased. Consistent with seasonal influenza and the 2009–2010 pandemic period, chronic medical conditions are important risk factors for severe disease during the winter season of 2010–2011. This study demonstrated the benefit of maintenance of severe patients surveillance to determine changes in the epidemiology of 2009 H1N1 infection after the pandemic period, and contributing to recommendations to target groups for influenza prevention and control interventions.