Pandemic Influenza (Human)

Unique identifier / Notation

BI0043

Synonyms

Pan flu.

An influenza pandemic is the worldwide spread of a new influenza virus to which there is little or no pre-existing immunity in the human population (WHO, 2021).

Primary reference(s)

WHO, 2021. World Health Organization (WHO). Accessed June 2021.

Additional scientific description

The constantly evolving nature of the influenza virus makes influenza among the top few infectious hazards with significant impact. A pandemic occurs when an influenza virus emerges to which there is little or no immunity in the global human population and which can transmit efficiently among people. The pandemic virus can be a virus strain jumping directly from animals or reassorted from animal viruses with or without human seasonal viruses.

Three influenza pandemics occurred at intervals of several decades during the 20th century, the most severe of which was the so-called ‘Spanish Flu’ (caused by an A(H1N1) virus), estimated to have caused 20–50 million deaths in 1918–2019. Milder pandemics occurred followed in 1957–1958 (‘Asian Flu’) caused by an A(H2N2) virus) and in 1968 (the ‘Hong Kong Flu’ caused by an A(H3N2) virus), which were estimated to have caused 1–4 million deaths each.

The current status of knowledge and technology means that predicting the next influenza pandemic – when, where, which virus strain, and how severe it will be – is impossible. Consequently, pandemic vaccines cannot be developed before the pandemic virus emerges. The World Health Organization (WHO) public health research agenda for influenza as an innovative research mechanism is key to inform and advance pandemic influenza preparedness (WHO, 2017a).

Meanwhile global influenza surveillance, through the WHO Global Influenza Surveillance and Response System (GISRS), assists with timely sharing of virus data and associated information, and national capacity building via seasonal influenza programs that are critical to mitigate the impact of the inevitable next pandemic (WHO, 2017b).

Metrics and numeric limits

The 2009 pandemic was caused by an influenza A (H1N1) virus. It is estimated to have caused between 100,000 and 400,000 deaths globally in the first year alone. Children and young adults were disproportionately affected in comparison to seasonal influenza, which causes severe disease mainly in the elderly, persons with chronic conditions and pregnant women (WHO, 2019a).

Key relevant UN convention / multilateral treaty

International Health Regulations (2005), 3rd ed. (WHO, 2016).

Examples of drivers, outcomes and risk management

Influenza pandemics, whether mild, moderate or severe, affect a large proportion of the global population, which puts significant strains on health and other essential services and may result in significant economic losses (WHO, 2019a).

Timely detection, characterisation and sharing of information about the pandemic virus directly affects the outcome of all downstream responses. The WHO GISRS is the foundation of such an operation. In an influenza pandemic, the virus is likely to spread rapidly. Rapid development, production and deployment of vaccines is critical to limit the potential impact on populations and essential services (WHO, 2019a).

Access to the appropriate vaccine in the early phases of a pandemic is greatly influenced by global production capacity and the lead-time required to produce influenza vaccines (WHO, 2019a).

As an influenza pandemic may last months or even years, it may require a sustained response in the health sector but also in other sectors providing essential services, such as energy and food production (WHO, 2019b).