Impact 5
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risk indicator
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Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


Pandemics are usually the result of a novel pathogen (virus, bacteria, fungi or other organisms that cause disease) emerging and spreading quickly around the world due to lack of population immunity. Once the pathogen emerges it is crucial to gain a rapid understanding of the spread, transmission, symptoms, severity, immunity, treatments and healthcare pathways. Experts consider a respiratory pathogen to be the most likely cause of a future pandemic affecting the UK based on the emergence of pandemics since 1900 and, as such, assumptions based upon a respiratory disease underpin the reasonable worst-case scenario for government pandemic preparedness planning. However, the government continues to plan and prepare for a range of pandemic and emerging infectious disease scenarios, across the five different transmission routes: respiratory, blood and sexual, touch, oral (food and water) and by vectors such as mosquitos. This approach covers known or unknown pathogens (referred to as "Disease X" by the World Health Organisation). The UK has flexible pandemic response capabilities that are built on lessons learned from exercises and incidents, including the COVID-19 pandemic.


The reasonable worst-case scenario is based on an unmitigated respiratory pandemic with an unassumed transmission route and a high attack rate, with 4% of symptomatic infections requiring hospital care and a case fatality ratio of 2.5%. From start to finish the emergency stage of the pandemic in the UK will last at least 9 months and potentially significantly longer. Response mechanisms are likely to be required beyond 9 months to manage the chronic stage of the risk and longer-term recovery. The pandemic may come in single or multiple waves. The wave number depends on the characteristics of the disease, public behaviour, and government intervention. The pandemic may lead to behaviour changes in the population depending on the nature of the disease and the government’s response.

The scenario assumes 50% of the UK’s population fall ill during the whole course of the pandemic, with about 1.34 million people estimated to require hospital treatment, possibly resulting in up to 840,000 deaths.

Key assumptions

Each pandemic is unique and will be impossible to predict when it will occur. Impacts on society depend on many different factors – transmission route, the time of year it emerges, severity of disease, global travel, who gets ill or dies and where it happens. For the purposes of the assessment, the scenario is an unmitigated pandemic that does not make any assumptions about behaviour change or government interventions being successful at reducing transmission.


Variations of the reasonable worst-case scenario for the pandemic risk, which is based on an influenza-like illness, are based on different pathogens, some of which have different routes of transmission. These include a possible novel enterovirus pandemic (these viruses are usually mild, but if they infect the central nervous system, they can cause serious illness); a novel coronavirus pandemic; and, a novel sexually transmitted infection pandemic.

Response capability requirements

Disease surveillance and early detection, including timely and reliable data, is needed. There should be procedures to support the identification and isolation of suspected cases and scalable contact tracing, scalable diagnostics (both lab and rapid testing), as well as rapid development and procurement of pharmaceutical countermeasures with stockpiled countermeasures, including personal protective equipment, for known pandemic threats. Effective non-pharmaceutical interventions, including border measures, should also be considered as part of the response.
Local and national plans for managing excess deaths should be present, and arrangements for effective UK and global coordination. Plans for social, educational, and economic impacts of the pandemic and expert scientific and clinical advice should also be in place.

Our response capability would need to be able to channel significant research and development resource to genomics and development of tests, vaccines and therapeutics. A national communications plan would also be needed to increase awareness and encourage good hygiene. Every sector, including but not limited to health and social care, will be affected by the pandemic and will require capabilities to respond.


It may take years for recovery to the health and social care sector (due to increased pressure on services throughout the pandemic) and impacts on society, education and the economy may last several years. Recovery from one wave of the pandemic may be hampered by the arrival of a subsequent wave of the same pandemic.