The number of people in hospital suffering from flu this summer was almost 20 times higher than in previous years, the latest figures show.

The impact of swine flu this summer caused the number of bed days spent in hospital — particularly among working-age men and women — to soar.

An estimated 100,000 new cases of swine flu were occurring weekly at the peak of the pandemic in July.

After a decline in rates of reported illness, cases have begun to increase. Last week, an estimated 84,000 new cases of the virus were reported, as experts predicted another spike in cases as the weather gets colder.

The overall number of patients in hospital — and the proportion of those requiring intensive care — also matched July’s rates.

More than 620,000 people are estimated to have fallen ill with the virus since its emergence in Britain in April.

Figures from the NHS Information Centre show that the number of bed days taken up by flu increased almost 20 times from 207 in July 2008 to 3,976 in July this year.

The latest figures also show that the proportion of people in different age groups needing hospital stays also changed because of swine flu.

In the three months to the end of July 2008, 40 per cent of bed days were taken up by children aged 16 and under but this fell to 31 per cent in the same period this year.

Bed days occupied by those aged 60 or over accounted for 40 per cent of the total in 2008 but just 22 per cent in 2009.

The biggest change was among working-age adults (people aged 17 to 59).

In the three months to the end of July 2008 they represented 19 per cent of all bed days but this rose to 46 per cent for the same months in 2009.

Commenting on today’s figures, Tim Straughan, chief executive of the NHS Information Centre, said that they revealed “the rapid increase in the demands on hospitals due to the flu virus in July this year.

“The peak in bed days taken up by flu in July 2009 is especially interesting as this is outside the normal flu season.

“In the preceding months, though increasing slowly, bed days attributed to flu were significantly lower.

“We will continue to monitor demand for beds to help the NHS gain a better understanding of the pressures caused by the current flu outbreak.”

Figures released last week by Sir Liam Donaldson, the Chief Medical Officer, showed that 848 patients in England were hospitalised on November 4, with 172 in intensive care.

The deaths of 154 people have been linked to the virus in Britain.

Although the majority of swine flu infections are mild, Sir Liam said that the numbers being treated in intensive care was “snowballing”, but he said it was not clear exactly why this was happening.

He emphasised that there was no evidence that the H1N1 swine flu virus was mutating to become more deadly.

“It is unexplained,” he said, adding that there was a similar trend in New Zealand during that country’s winter flu season.

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