Assumptions for Planning
Influenza H1N1 ("swine flu") appeared on the global health scene in April 2009, and by June it had become a pandemic.
The virus was swine in origin, not avian. The outbreak began in the Americas, not Asia. These facts were contrary to the best guesses of renowned flu experts worldwide - including those at the World Health Organization (WHO).
Everyone had an eye on avian flu H5N1 as the virus with the strongest pandemic potential. And indeed, it remains a viable threat. WHO cautioned the world to remain vigilant against avian flu even as it announced that H1N1 had become a pandemic.
The surprising origins and evolutions of the 2009 pandemic illustrate perfectly what responsible pandemic experts always stress: pandemic preparedness is, to some degree, guesswork. No one knows when pandemics will occur, which virus will be responsible, how great the impact will be, or how long it will last.
And yet, a lack of knowledge is not an excuse to remain unprepared. For planning purposes, pandemic experts agree on a baseline from which to work. Prior to April 2009, the impact of the next pandemic was modelled on previous influenza pandemics of the twentieth century.
Several national authorities have provided assumptions for planning purposes. They emphasise that these assumptions are NOT predictions, but should be used to plan for "reasonable worst case" scenarios.