Influenza-attributable disease burden in outpatients with influenza-likeillness in China

Abstract

BACKGROUND: Human influenza virus infections cause a considerable burden of morbidity and mortality worldwide each year. The virus is associated with a large number of excess deaths, hospitalizations and outpatient visits as well as absences from work resulting from infections. Population-based estimates of influenza-associated excess hospitalization and death have been increasingly reported. However few data have been published on influenza-attributable excess outpatient consultations that may also bring considerable economic burden to the society. METHOD: Sentinel surveillance data on virologic confirmation of sentinel specimens and outpatient visits due to influenza-like-illness (ILI) and other diseases was obtained for each province and municipality of China in 2005-2013. Generalized additive regression models were fitted to weekly ILI outpatient consultations to estimate weekly influenza-associated excess ILI outpatient visits for each individual province, accounting for seasonal baselines and other potential confounders. The influenza-associated excess ILI outpatient visits were estimated as the difference between the predicted ILI consultations from the model based on reported detection of influenza virus and the predicted ILI consultations by setting the virus activity as zero. RESULTS: Overall influenza was associated with 3.9 excess ILI consultations per 1,000 persons in 30 provinces (data not available for Tibet) of China each year from 2005 to 2013. The highest annual ILI burden was observed in Tianjin municipality, Shanghai municipality, Guangdong province and Zhejiang province, ranging from 8.4 to 20.0 per 1,000 population (exclude 2009). Influenza A(H1N1)pdm09 led to a higher number of influenza-associated ILI consultations in 2009 across all provinces compared to other years. Seasonal influenza A(H3N2) and A(H1N1)pdm09 were associated with a relatively higher number of ILI consultations than seasonal A(H1N1) and B. The excess ILI burden was 9.1 per 1,000 population among children aged below 5 years old, substantially higher than in adults. CONCLUSION: Human influenza viruses caused considerable impact on population morbidity, potentially leading to a substantial economic burden. Variations of excess ILI burden associated with different virus types/subtypes and age groups may provide evidence for planning of vaccination programmes in the future in order to reduce the public health impact of influenza.