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

Influenza is a common disease that affects the respiratory tract having high incidence rates around different nations in the world. In recent years, there has been an upsurge in national efforts to assess the burden of influenza disease, notably after the release of the WHO handbook on calculating the disease burden associated with seasonal influenza. A systematic literature review estimating influenza- associated hospitalization between 1995-2020 was done. The study included an assessment of the impact of five study factors on hospitalisation.

Read more to know about this estimation and the impact of seasonal influenza epidemics on hospitalisation rates.

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 Chinese-language studies published between 1995 and 2020 estimating influenza-associated hospitalisation. We included a total of 127 studies (seven in Chinese) 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 envelopes: narrow, medium, or wide), 3) whether every case was laboratory-confirmed 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 40.5 (95% confidence interval (CI)=24.3-67.4) per 100000 persons, with rates varying substantially by age: 224.0 (95% CI=118.8-420.0) in children aged 0-4 years and 96.8 (95% CI=57.0-164.3) in the elderly aged >65 years. The overall pooled hospitalisation rates varied by calculation method; for all ages, the rates were significantly 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 100000 in New Zealand to 122.1 (95% CI=41.5- 358.4) per 100000 in India (all age estimates).

Conclusion

Using the pooled global influenza-associated hospitalisation rate, we estimate that seasonal influenza epidemics result in 3.2 million cases of severe illness (hospitalisations) per annum. More extensive analyses are required to assess the influence of other factors on the estimates (e.g. vaccination and dominant virus (sub)types) and efforts to harmonize the methods should be encouraged. Our study highlights the high rates of influenza-associated hospitalisations in children aged 0-4 years and the elderly aged 65+ years.

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