Title Concurrent comparison of epidemiology , clinical presentation and outcome between adult patients suffering from the pandemic influenza A ( H 1 N 1 ) 2009 virus and the seasonal influenza A virus infection

Purpose of study The demographics, clinical features and outcome of patients with pandemic influenza A (H1N1) 2009 infection were compared with a concurrent cohort of patients with seasonal influenza A infection. Study design The clinical and microbiological data of hospitalised adult patients admitted between 29 June and 28 October 2009, with pandemic A (H1N1) 2009 or seasonal influenza A infection, were analysed. Results A total of 186 patients including 69 pandemic A (H1N1) and 117 seasonal influenza were analysed. The majority (75%) under 50 years of age had pandemic A (H1N1). Compared with seasonal influenza, pandemic A (H1N1) patients were younger (median age 47 years vs 76 years, p<0.001), less likely to have lower respiratory tract symptoms (46.4% vs 66.7%, p1⁄40.007), but more likely to be obese (5.8% vs 0%, p1⁄40.018), pregnant (7.2% vs 0.9%, p1⁄40.027) or have no underlying predisposing factors (24.6% vs 5.1%, p<0.001). Patients with pandemic A (H1N1) were more likely to receive oseltamivir (91.3% vs 40.2%, p<0.001), but less likely to receive antibiotics (75.4% vs 90.6%, p1⁄40.005). Respiratory failure was the reason for intensive care unit admission for all four patients with pandemic A (H1N1), but only for one of three patients with seasonal influenza. There were no statistical significant differences in the rate of intensive care unit admission or death. Conclusions In addition to age, several clinical parameters were different between pandemic A (H1N1) and seasonal influenza. However, since both seasonal and pandemic influenza can lead to significant morbidity and mortality, the impact of pre-existing seasonal influenza should not be underestimated during the pandemic period. INTRODUCTION Pandemic influenza has been considered to cause greater morbidity and mortality than seasonal influenza, particularly among young adults. In Mexico, the rate of severe pneumonia during the initial period of pandemic A (H1N1) 2009 was found to be much higher than in preceding years, with deaths predominantly occurring in the younger age group. Another alarming feature of the current pandemic influenza is that >30% of patients with critical illness had no known predisposing conditions. A few studies involving paediatric patients have undertaken a more detailed comparison between the pandemic A (H1N1) 2009 and seasonal influenza. These studies have identified older children and those with underlying diseases to be found more frequently among hospitalised patients with pandemic A (H1N1) than those with seasonal influenza. 5 The death rate of pandemic A (H1N1) among children and pregnant women is also higher than that of seasonal influenza from previous years. 7 The major limitation of these studies is that historical data were used for comparison. Potential biases may arise when historical controls were used. For example, the threshold for hospital admission may vary during the pandemic and inter-pandemic period. There have been two studies in the southern hemisphere which concurrently compared pandemic A (H1N1) and seasonal influenza. The first study, conducted in Sydney, Australia, found that patients with pandemic A (H1N1) were younger and less likely to be immunocompromised than those with seasonal influenza. In that study, there were no statistically significant differences in the clinical features and outcome between pandemic A (H1N1) and seasonal influenza. In contrast, a study involving eight intensive care units (ICUs) in New South Wales, Australia, showed that the relative risk of admission to ICU of patients with pandemic A (H1NI) was 4.9 times higher than for those with seasonal influenza A infection. Pandemic A (H1N1) was first introduced into Hong Kong on 30 April 2009. In this study, we sought to compare concurrently the epidemiology, clinical features, and outcome of hospitalised patients with pandemic and seasonal influenza. We chose to investigate patients hospitalised during the mitigation phase (started from 29 June 2009) to avoid including hospital admissions for isolation purpose only during the preceding containment phase of the pandemic. METHODS Patients This retrospective study was approved by the institutional review board of the Hospital Authority of Hong Kong. Patients with laboratory confirmed influenza A virus infection were identified by the laboratory information system. Inclusion criteria were age 18 years or above; admission to our hospital during the 4 month period between 29 June to 28 October 2009; and positive reverse transcriptase-polymerase chain reaction (RT-PCR) Infectious Disease Division, Queen Mary Hospital, State Key Laboratory of Emerging Infectious Diseases, Carol Yu Centre for Infection, The University of Hong Kong, Hong Kong Special Administrative Region, China Correspondence to Professor Kwok-Yung Yuen, Carol Yu Centre for Infection and Division of Infectious Diseases, Department of Microbiology, The University of Hong Kong, Queen Mary Hospital, Pokfulam Road, Pokfulam, Hong Kong Special Administrative Region, China; kyyuen@hkucc.hku.hk Received 16 December 2009 Accepted 17 May 2010 Published Online First 5 August 2010 Postgrad Med J 2010;86:515e521. doi:10.1136/pgmj.2009.096206 515 Original article group.bmj.com on May 12, 2011 Published by pmj.bmj.com Downloaded from