Scottish Government

Annex B

Main Planning Assumptions About a Human Flu Pandemic (from UK Influenza Pandemic Contingency Plan)

• A new pandemic will be due to a new sub-type of influenza A;
• A pandemic strain could emerge anywhere, including the UK, but is most likely to emerge in China or the Far East; spread from an origin in Asia is likely to follow the main routes of travel and trade;
• In the event of a novel influenza virus causing significant outbreaks of human illness elsewhere in the world, it is unlikely that the UK could prevent importation (except by closing all borders); even a 99.9% restriction on travel into the country would only be expected to delay importation of the virus by up to two months;
• Spread from the source country to the UK is likely to take around a month…modern travel may result in wide international spread even more rapidly than this;
• Following arrival in the country it will take a further 2-3 weeks until cases are occurring across the whole country;
• Once influenza levels exceed the baseline threshold, influenza activity in the UK may last for 3-5 months, depending on the season, and there may be subsequent waves, weeks or months apart;
• All ages will be affected, but children and otherwise fit adults could be at relatively greater risk, particularly should elderly people have some residual immunity from exposure to a similar virus earlier in their lifetime;
• For planning purposes the base scenario, based on previous pandemics in the 20th century, is a cumulative clinical attack rate of 25% of the population 9the figure advised by the WHO) over one or more waves of around 15 weeks each, weeks or months apart. The second wave may also be the more severe. 10% and 50% attack rates have also been considered;
• The impact of overall case fatality rates between 0.37% and 2.5% have also been considered. See the table below

Range of possible excess deaths, UK population