Revisiting influenza-hospitalisation estimates from the Burden of Influenza and Respiratory Syncytial Virus Disease (BIRD) project using different extrapolation methods

The World Health Organization (WHO) has estimated that globally, seasonal influenza epidemics result in three to five million cases of severe illness (hospitalisations) every year [1,2]. In a recent paper in the Journal of Global Health, the Burden of Influenza and Respiratory Syncytial Virus Disease (BIRD) project aimed to improve the understanding of influenza-associated hospitalisation estimates at national and global levels [3]. We explored the heterogeneity of the country estimates obtained from the literature by taking into account the role of five study design factors - calculation method, outcome measure, presence of laboratory confirmation, national or sub-national data, and use of single or multi-year data. When we applied this overall estimated pooled rate (40.5 per 100 000 population; 95% confidence interval (CI) = 24.3-67.4) to the world population, we estimated there were, on average, 3.2 million (95% CI = 1.9-5.3) influenza-associated hospitalisations per year [3]. Other research groups have also developed annual global hospitalisation estimates, and these vary widely from 3.2 million based on the BIRD project [3] to 9.46 million (95% CI = 3.7-22.9) estimated from the Global Burden of Disease Study 2017 [4].