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      Influenza-Associated Hospitalization in a Subtropical City

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          Abstract

          Background

          The impact of influenza on morbidity and hospitalization in the tropics and subtropics is poorly quantified. Uniquely, the Hong Kong Special Administrative Region has computerized hospital discharge diagnoses on 95% of total bed days, allowing disease burden for a well-defined population to be accurately assessed.

          Methods and Findings

          Influenza-associated morbidity and hospitalization was assessed by Poisson regression models for weekly counts of hospitalizations in Hong Kong during 1996 to 2000, using proportions of positive influenza types A (H1N1 and H3N2) and B isolations in specimens sent for laboratory diagnosis as measures of influenza virus circulation. We adjusted for annual trend, seasonality, temperature, and relative humidity, as well as respiratory syncytial virus circulation. We found that influenza was significantly associated with hospitalization for acute respiratory disease (International Classification of Diseases version 9 codes [ICD9] 460–466 and 480–487) and its subcategory pneumonia and influenza (ICD9 480–487) for all age groups. The annual rates of excess hospitalization per 100,000 population for acute respiratory diseases for the age groups 0–14, 15–39, 40–64, 65–74, and 75+ were 163.3 (95% confidence interval [CI], 135–190), 6.0 (95% CI, 2.7–8.9), 14.9 (95% CI, 10.7–18.8), 83.8 (95% CI, 61.2–104.2), and 266 (95% CI, 198.7–330.2), respectively. Influenza was also associated with hospitalization for cerebrovascular disease (ICD9 430–438) for those aged over 75 y (55.4; 95% CI, 23.1–87.8); ischemic heart disease (ICD9 410–414) for the age group 40–64 y (5.3; 95% CI, 0.5–9.5) and over 75 y (56.4; 95% CI, 21.1–93.4); and diabetes mellitus (ICD9 250) for all age groups older than 40 y.

          Conclusions

          Influenza has a major impact on hospitalization due to cardio-respiratory diseases as well as on cerebrovascular disease, ischemic heart disease, and diabetes mellitus in the tropics and subtropics. Better utilization of influenza vaccine during annual epidemics in the tropics will enhance global vaccine production capacity and allow for better preparedness to meet the surge in demand that is inevitable in confronting a pandemic.

          Abstract

          Influenza was found to have a major impact on hospitalization in Hong Kong due to respiratory diseases, cardiovascular disease, and diabetes. Previously, little was known about this impact in the tropics and subtropics.

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          Most cited references34

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          The effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children.

          Despite high annual rates of influenza in children, influenza vaccines are given to children infrequently. We measured the disease burden of influenza in a large cohort of healthy children in the Tennessee Medicaid program who were younger than 15 years of age. We determined the rates of hospitalization for acute cardiopulmonary conditions, outpatient visits, and courses of antibiotics over a period of 19 consecutive years. Using the differences in the rates of these events when influenzavirus was circulating and the rates from November through April when there was no influenza in the community, we calculated morbidity attributable to influenza. There was a total of 2,035,143 person-years of observation. During periods when influenzavirus was circulating, the average number of hospitalizations for cardiopulmonary conditions in excess of the expected number was 104 per 10,000 children per year for children younger than 6 months of age, 50 per 10,000 per year for those 6 months to less than 12 months, 19 per 10,000 per year for those 1 year to less than 3 years, 9 per 10,000 per year for those 3 years to less than 5 years, and 4 per 10,000 per year for those 5 years to less than 15 years. For every 100 children, an annual average of 6 to 15 outpatient visits and 3 to 9 courses of antibiotics were attributable to influenza. In winter, 10 to 30 percent of the excess number of courses of antibiotics occurred during periods when influenzavirus was circulating. Healthy children younger than one year of age are hospitalized for illness attributable to influenza at rates similar to those for adults at high risk for influenza. The rate of hospitalization decreases markedly with age. Influenza accounts for a substantial number of outpatient visits and courses of antibiotics in children of all ages.
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            Influenza and the rates of hospitalization for respiratory disease among infants and young children.

            Young children may be at increased risk for serious complications from influenzavirus infection. However, in population-based studies it has been difficult to separate the effects of influenzavirus from those of respiratory syncytial virus. Respiratory syncytial virus often circulates with influenzaviruses and is the most frequent cause of hospitalization for lower respiratory tract infections in infants and young children. We studied the rates of hospitalization for acute respiratory-disease among infants and children during periods when the circulation of influenzaviruses predominated over the circulation of respiratory syncytial virus. For each season from October to May during the period from 1992 to 1997, we used local viral surveillance data to define periods in Washington State and northern California when the circulation of influenzaviruses predominated over that of respiratory syncytial virus. We calculated the rates of hospitalization for acute respiratory disease, excess rates attributable to influenzavirus, and incidence-rate ratios for all infants and children younger than 18 years of age who were enrolled in either the Kaiser Permanente Medical Care Program of Northern California or the Group Health Cooperative of Puget Sound. The rates of hospitalization for acute respiratory disease among children who did not have conditions that put them at high risk for complications of influenza (e.g., asthma, cardiovascular diseases, or premature birth) and who were younger than two years of age were 231 per 100,000 person-months at Northern California Kaiser sites (from 1993 to 1997) and 193 per 100,000 person-months at Group Health Cooperative sites (from 1992 to 1997). These rates were approximately 12 times as high as the rates among children without high-risk conditions who were 5 to 17 years of age (19 per 100,000 person-months at Northern California Kaiser sites and 16 per 100,000 person-months at Group Health Cooperative sites) and approached the rates among children with chronic health conditions who were 5 to 17 years of age (386 per 100,000 person-months and 216 per 100,000 person-months, respectively). Infants and young children without chronic or serious medical conditions are at increased risk for hospitalization during influenza seasons. Routine influenza vaccination should be considered in these children.
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              Methods for current statistical analysis of excess pneumonia-influenza deaths.

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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                PLoS Med
                pmed
                PLoS Medicine
                Public Library of Science (San Francisco, USA )
                1549-1277
                1549-1676
                April 2006
                7 March 2006
                : 3
                : 4
                : e121
                Affiliations
                [1] 1University of Hong Kong, Department of Community Medicine, Hong Kong, Hong Kong Special Administrative Region, China
                [2] 2University of Hong Kong, Department of Microbiology, Queen Mary Hospital, Hong Kong, Hong Kong Special Administrative Region, China
                Royal Free and University College Medical School United Kingdom
                Author notes
                * To whom correspondence should be addressed. E-mail: malik@ 123456hkucc.hku.hk

                Author Contributions: Author contributions and additional contributors are listed at the end of this paper.

                Competing Interests: The authors have declared that no competing interests exist.

                Article
                10.1371/journal.pmed.0030121
                1391978
                16515368
                5c497bba-dd97-460f-ac05-bafaccff59a7
                Copyright: © 2006 Wong et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                History
                : 14 October 2005
                : 21 December 2005
                Categories
                Research Article
                Cardiology/Cardiac Surgery
                Diabetes/Endocrinology/Metabolism
                Epidemiology/Public Health
                Health Policy
                Respiratory Medicine
                Infectious Diseases
                Respiratory Medicine
                Diabetes
                Cardiovascular Medicine
                Health Policy

                Medicine
                Medicine

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