Almost a quarter of patients in critical care with swine flu are children and doctors are “unnerved” by the severity of the disease, the Chief Medical Officer reported yesterday.

Sir Liam Donaldson said that the number of patients in critical care in England had risen from 74 to 99 in the past week, with 23 of them aged under 16. He warned that, although the H1N1 virus had not mutated into a more severe form, doctors were “very unnerved by the severity of illness and how life-and-death situations arose very early”. He added: “The speed of development [of the disease] has been very rapid in some cases.”

Describing the rise in critical care rates, which are higher than at any point in the pandemic, Sir Liam said: “We don’t understand why this has happened. It looks as if the virus is having a different impact in the flu season than it had in the summer.”

New planning guidance issued by the Department of Health suggests the need to prepare for 12 per cent of the population to contract swine flu, and emphasises that up to a third of children could become infected.

The advice was issued as latest figures from the Health Protection Agency (HPA) showed a doubling of weekly infections to 53,000 in England. A further 16 deaths across Britain were confirmed this week, bringing to 122 the number of people who have died after contracting the virus. A third had either mild or no underlying health problems.

Sir Liam warned that the rise in severe cases would put increased pressure on hospital beds. “We are starting to worry about the sustained pressure over the winter that the NHS might face. It’s a very long period for the NHS to sustain its response.”

He also reiterated his concern at the high proportion of deaths in younger age groups, describing it as a “very unusual pattern for flu”. More than half (54 per cent) of deaths have been in the under-45 age group while 77 per cent have been in the under-65 group. With seasonal flu, most deaths would be expected in the over-65s.

In some areas, more than 15 per cent of paediatric critical-care beds are occupied by patients with swine flu. Sir Liam’s comments came as Nicola Sturgeon, the Scottish Health Secretary, disclosed that four people had died from swine flu in Scotland in the past 24 hours, including a teenage boy. All had underlying health problems.

The latest HPA estimates suggest that a total of 435,000 people have contracted the H1N1 virus so far. The updated planning guidance, which is being issued to the NHS, suggests that the number of cases could peak at 1.5 million people in a single week.

It also advises planning for 35,000 more hospital admissions, with 15 per cent of patients needing critical care. If such a scenario unfolds, there could be a total of 1,000 deaths and 5 per cent of workers off sick.

At the weekly swine-flu briefing at the Department of Health in London, Ian Dalton, national director of NHS Flu Resilience, said the health service had put an “unprecedented amount” of work into preparing for the virus and to being able to increase the number of critical-care beds where necessary. “The closer we get to full capacity the more likely it is that we would need to undertake the surge measures to increase levels of critical-care beds,” he said. “Surge” levels involve increasing capacity by 100 per cent.

A national swine flu immunisation campaign was launched on Wednesday. Frontline health workers and hospital patients deemed at risk, such as pregnant women and cancer sufferers, were the first to receive the vaccine.

GPs will begin immunising at-risk groups in the community, including pregnant women and people with chronic conditions such as heart disease and diabetes, from Monday. In the coming months, 12 million people will be offered the jab as a priority.

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