Estimating seasonal influenza burden across the disease severity pyramid

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14 October 2022 WHO Departmental news

WHO and Johns Hopkins University have developed a webtool that allows countries with estimates of influenza-associated hospitalization or death to generate influenza disease burden estimates across the severity pyramid, from moderate disease to death.

Most low- and middle-income countries with influenza surveillance systems collect and report data on hospitalizations and, sometimes, deaths. But few distinguish between mild, moderate non-hospitalized cases and critically ill hospitalized cases. Quantifying the burden of influenza across the disease severity spectrum is important to understand and communicate the full impact of seasonal influenza on health systems and societies. With support of the PIP Partnership Contribution and in collaboration with the Johns Hopkins Center for Health Security, WHO has developed a user-friendly webtool to facilitate the estimation of seasonal influenza burden across the disease severity pyramid, from mild/moderate non-hospitalized cases to influenza-associated deaths with a focus on medically attended illness.

The tool, which is called the Seasonal Influenza Burden of Disease Estimator, uses a multiplier-based approach that allows countries with data on influenza-associated hospitalizations or deaths to estimate seasonal influenza burden at all levels of the disease severity pyramid. For example, a country with estimates of influenza-associated hospitalizations can use the tool to generate estimates of deaths and mild/moderate cases.

To set parameters for the tool, the research team first did a literature review to identify age-specific multipliers and ratios of influenza-associated hospitalizations to deaths, hospitalized critically ill cases, and mild/moderate non-hospitalized cases. The range of multipliers (minimum and maximum) found in the literature are used to bound uncertainty in model-generated estimates. Users can also use their own custom multipliers in the tool. The team validated the tool using real-world data and is now pilot testing it in select countries.

The tool’s output improves understanding of the impact of seasonal influenza and allows public health practitioners to communicate the burden of seasonal influenza across levels of severity to policy-makers. It will be refined based on feedback from the pilot testing in countries, and potentially expanded to incorporate an estimate of the burden averted through influenza vaccination programs. Future directions may include using the tool’s output to generate cost-effectiveness estimates for vaccination or other interventions.

Access the tool at www.flutool.org