The updated algorithm comes as the number of confirmed cases in the UK rises to 68.
The three new cases are two children and one adult from the London area, all close contacts of previously confirmed cases.
Elsewhere, experts from Imperial College London have warned that a full-scale pandemic could result in a third of the world’s population being infected with swine flu.
The WHO estimates that there are currently 5,251 confirmed cases of swine flu in some 30 countries around the world.
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