Excess hospitalization associated with seasonal and pandemic influenza in a subtropical city

Abstract

BACKGROUND: It is critical to obtain reliable estimates of the morbidity burden of pandemic influenza in order to properly assess the health impact of the 2009 pandemic. To date there are no studies that used a standardized modeling approach to assess the age- and cause-specific hospitalization associated with different types or subtypes of influenza viruses. METHODS: We fitted Poisson regression models to weekly numbers of cause-specific hospitalization in Hong Kong. Excess hospitalization associated with seasonal and pandemic influenza from 2005 to 2010 was simultaneously calculated from the models with the proxy variable of proportions of specimens positive for pandemic H1N1, seasonal influenza A (subtypes H3N2 and H1N1), and influenza B. Seasonal trend, meteorological factors, and co-circulating viruses were also adjusted for as confounders in the model. RESULTS: Age-standardized annual excess rates of hospitalization associated with influenza were markedly higher in 2009 and 2010 than those in the other years. In children and young adults, higher hospitalization rates of ARD were found attributable to pandemic H1N1 than those attributable to seasonal influenza, whereas in the elderly the hospitalization rates attributable to pandemic H1N1 were lower than those attributable to 2 subtypes of seasonal influenza A. For patients with chronic diseases including cardiovascular disease, ischemic heart disease, stroke, and diabetes, excess rates of hospitalization associated with pandemic H1N1 were higher than those associated with seasonal influenza in both young and old adults. Comparison of our cause-specific estimates with the numbers of reported hospitalized cases confirmed with pandemic infections by laboratory tests showed that the morbidity burden of the 2009 H1N1 pandemic was seriously underreported in the elderly and people with chronic conditions. CONCLUSIONS: The hospitalization risks associated with influenza were dramatically elevated in the 2009 H1N1 influenza pandemic, particularly in the age groups younger than 65 years. Underreporting of hospitalized pandemic cases in the elderly and those with chronic conditions suggests a need to enhance surveillance in these groups.