Models are being developed and updated continuously to ensure that the NHS has the capacity to cope as best it can with the spread of swine flu.

The latest estimates, calculated for the first wave of the pandemic running to the new year, offer a “worst-case scenario” to ensure that all possibilities are catered for and that the pressure points, such as emergency care and staffing rotas, are as strong as possible.

While the Government emphasises that the suggestion of a range of between 19,000 and 65,000 deaths is not a prediction of how the pandemic will develop, it is nevertheless based on the current rate of infection. Should this continue, a substantial wave of cases, with up to 30 per cent of the population experiencing symptoms, could peak in early September.

Another possibility is that seasonal effects might slow the spread in coming months with a resurgence likely when children go back to school, leading to a pandemic peak in October or even later.

The figure of 65,000 deaths is arrived at from a calculation based on a fatality rate of up to 0.35 per cent of all clinical cases: if 30 per cent of the population falls ill — roughly 20 million people — and 0.35 per cent of these die, the death toll reaches such a figure. Should the proportion of people with symptoms be lower (the bottom end of the range suggests as little as 5 per cent by September), then the death estimates will fall substantially.

In both the recent pandemics, of 1957-58 and 1968-70, between 25 and 30 per cent of people were infected. A total of 33,000 died during the former, and a few thousand less in the latter.

Of more importance to NHS preparations at a local level — and the service’s overall effectiveness countrywide — are rates of complication and hospitalisation, the ages affected and the geographical spread on a month by month basis.

Under the plans, the weekly “clinical attack rate” for a local area — the number of people affected in one community — is about 8 per cent. As evidence already emerging from around the country suggests, some areas may face sharp peaks, and others a slower spread of the virus.

Last week family doctors in Tower Hamlets had 759 consultations for swine flu per 100,000 population, double the next worst affected area (nearby City & Hackney). Yorkshire and the Humber remains relatively untouched, despite high rates in regions around it.

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