Pandemic influenza preparedness is essential. By applying, evaluating and refining tools to measure the severity of seasonal influenza epidemics every year, WHO and Member States can be better prepared. WHO aims to provide a structured approach for countries to visualize and interpret data. This approach offers the scientific evidence necessary to communicate risk and inform the timing, scale, emphasis, intensity and urgency of response actions. The pandemic and seasonal influenza severity assessment (PISA) updated guidance seeks to achieve precisely this goal, ensuring that data visualization and interpretation lead to effective decision-making and response planning.
The aims of the influenza severity assessment at the national level are to:
- describe the epidemiological situation in the context of historical data and assess the severity of an influenza epidemic or pandemic based on all reliable information;
- inform national and global risk assessments; and
- apprise public health preparedness, response and recovery measures, as well as resource allocation.
PISA involves using historical data to set thresholds which define levels of activity ranging from "below baseline" through to "extraordinary". By comparing current activity to these thresholds, countries can put not only pandemics but also regular seasonal epidemics into perspective.
WHO has recently published the second edition of the PISA guidance (the first edition was published in 2017). This revision takes into account changes in respiratory virus surveillance, as well as lessons learned by countries during the COVID-19 pandemic. It was developed in alignment with WHO initiatives and recommendations such as the WHO Global Influenza Strategy (2019-2030), the Preparedness and Resilience to Emerging Threats (PRET) initiative, the Mosaic Respiratory Surveillance Framework, and the WHO interim guidance for the integration of SARS-CoV-2 and influenza virologic surveillance.
Updates to the guidance broaden the scope of PISA so that the methodology can be applied to syndromic data (such as influenza-like illness (ILI) and severe acute respiratory infections (SARI)) as well as influenza-confirmed activity (such as influenza-confirmed ILI or influenza-confirmed SARI). The PISA approach can also be applied to assess the activity of other epidemic-prone respiratory viruses, such as respiratory syncytial virus (RSV) and SARS-CoV-2, as well as future respiratory viruses of pandemic potential.
The impact of epidemics on healthcare capacity has been added as a new indicator to increase the utility of PISA thresholds for assessing the need for public health and social measures. There are now four indicators that countries can select to describe different aspects of severity using their existing surveillance data: transmissibility, seriousness of disease, morbidity and mortality and impact on healthcare capacity.
The guidance was developed through the continuing contributions of the PISA technical working group and a consultation meeting in June 2023 with external experts from all six WHO regions as well as WHO staff from headquarters and regional offices.
Between March and June 2024, in-person training workshops on the new guidance have been conducted in seven countries (Bangladesh, Bhutan, Egypt, Ethiopia, Morocco, Oman and the United Republic of Tanzania), with further workshops planned.
