Global and national influenza-associated hospitalisation rates: Estimates for 40 countries and administrative regions

Background WHO estimates that seasonal influenza epidemics result in three to five million cases of severe illness (hospitalisations) every year. We aimed to improve the understanding of influenza-associated hospitalisation estimates at a national and global level.Methods We performed a systematic literature review of English-and Chi-nese-language studies published between 1995 and 2020 estimating influen-za-associated hospitalisation. We included a total of 127 studies (seven in Chi-nese) in the meta-analysis and analyzed their data using a logit-logistic regression model to understand the influence of five study factors and produce national and global estimates by age groups. The five study factors assessed were: 1) the method used to calculate the influenza-associated hospitalisation estimates (rate-or time series regression-based), 2) the outcome measure (divided into three en-velopes: narrow, medium, or wide), 3) whether every case was laboratory-con-firmed or not, 4) whether the estimates were national or sub-national, 5) whether the rates were based on a single year or multiple years.Results The overall pooled influenza-associated hospitalisation rate was 40.1 (95% confidence interval (CI) = 23.3-69.1) per 100 000 persons, with rates vary-ing substantially by age: 137.8 (95% CI = 70.6-268.7) in children aged 0-4 years and 71.6 (95% CI = 39.9-127.7) in the elderly aged >65 years. The overall pooled hospitalisation rates varied by calculation method; for all ages, the rates were sig-nificantly higher when they were based on rate-based methods or calculated on a single season and significantly lower when cases were laboratory-confirmed. The national hospitalisation rates (all ages) varied considerably, ranging from 11.7 (95% CI = 3.8-36.3) per 100 000 in New Zealand to 122.1 (95% CI = 41.5-358.4) per 100 000 in India (all age estimates).Conclusions Using the pooled global influenza-associated hospitalisation rate, we estimate that seasonal influenza epidemics result in 3.2 million cases of se-vere illness (hospitalisations) per annum. More extensive analyses are required to assess the influence of other factors on the estimates (e.g. vaccination and dom-inant virus (sub)types) and efforts to harmonize the methods should be encour-aged. Our study highlights the high rates of influenza-associated hospitalisations in children aged 0-4 years and the elderly aged 65+ years.

Automated summary

This study aimed to improve the understanding of influenza-associated hospitalisation estimates by conducting a systematic literature review and meta-analysis of studies published between 1995 and 2020. The results showed significant variations in hospitalisation rates by age, with higher rates observed in children aged 0-4 years and the elderly aged >65 years. The study highlights the need for further analysis of other factors and harmonization of methods.