Swine flu is spreading so rapidly that the Government cannot contain it any more. The number of new cases could reach 100,000 a day by the end of August, the Health Secretary Andy Burnham warned yesterday.

Firms are being warned to prepare to have up to one-fifth of their staff calling in sick over the autumn. It is also inevitable that the number of deaths from the virus will rise. So far, there have been 7,447 confirmed cases of swine flu in the UK, but only three patients have died and all of those had underlying health problems.

If the death toll stays at that level (roughly one in 2,500), the disease could be killing 40 people a day by September – but the Chief Medical Officer, Sir Liam Donaldson, said yesterday that there was not enough information to make precise predictions.

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Mr Burnham told MPs: "We have always known it would be impossible to contain the virus indefinitely and at some point we would need to move away from containment to treatment. Cases are doubling every week and on this trend we could see over 100,000 cases per day by the end of August. Scientists can expect to see rapid rises in the number of cases."

The Department of Health stressed yesterday that, although the virus is now effectively beyond their control, there is no reason to think that it is any more deadly than before – although there are fears that it could mutate into a more virulent form. While a few sufferers have been seriously ill with the disease, others are believed to have had a mild attack without even knowing they have swine flu.

Those who have gone down with the virus are advised to see a doctor and stay at home until they feel better, but the Department of Health said yesterday that there is no general advice on how long that will take, as it depends on the severity of the attack.

The fact that the Government is not trying to contain the disease any longer means that the NHS will stop the practices of issuing the drug Tamiflu to every known sufferer, and tracing people who have come into contact with it and giving them Tamiflu as a preventative measure. The practice has been criticised as an extravagant use of resources that risks shortening the time it will take for the virus to develop a strain that can resist Tamiflu. "We've been fighting this pandemic very aggressively – we're unapologetic about that," said Sir Liam.

The change in policy also means that schools where a pupil or member of staff has contracted swine flu will not have to close, other than in exceptional circumstances.

The Government will also abandon its daily log of the exact numbers who have contracted the disease, replacing it with "more general" estimates. Instead of laboratory testing for swine flu, patients will be diagnosed by GPs or by NHS Direct, relieving pressure on health laboratories. If doctors believe the person is suffering from swine flu they will be told to stay at home and be given a voucher which a "flu friend" or family member can take to a drug collection point, such as a pharmacy

The Government has ordered enough flu vaccines to cover the whole population of the UK. Mr Burnham said they should become available from next month, with 60 million doses – enough for 30 million people – available by the end of the year. Others have questioned whether they can be produced that quickly.

At the press conference, Mr Burnham emphasised that the 100,000 figure was "a projection, not a fact". He added: "It's very important for that figure to be reported for what is – a projection, nothing else."

The projection is based on the fact that the number of reported cases has more than doubled from just under 3,600 to nearly 7,500. Yesterday's figures show that those most likely to contract swine flu are children aged five to 14, and that the main outbreaks have been in the South and the Midlands. No part of the UK is free of the disease.

Hundreds of thousands of employees are expected to call in sick as the weather gets colder. John Cridland, CBI deputy director general, said: "Many firms already have contingency plans in place to deal with emergencies, including a flu pandemic. All firms need to ensure they are prepared if swine flu becomes more serious in the UK as the year goes on. This may involve introducing measures such as remote working in order to deal with possible disruption to normal operations."

Samuel Ashbridge: 'He is absolutely fine but bored stiff'

Samuel Ashbridge's parents had the call from school at lunchtime on Monday that their 11-year-old son was ill. He was fine when he left home, but the symptoms of swine flu, including a high temperature, sickness and a cough, overcame him around noon.

The previous week, the whole of his year at the City of London Freemen's school, Surrey, had been on a school trip. By Monday, 22 of the 60 children on that trip had been taken ill.

"We rang NHS Direct, who put us through a lot of checks," his father, Ian Ashbridge said. "We told them the symptoms, and they said we should call our doctor. The doctor put him on Tamiflu.

"The doctor came about 7.30 that evening with a kit in a blue plastic bag, took some swabs and sent them off. We haven't actually got our results, but he is absolutely fine, bored stiff and wanting to go back to school, while the lab are still umm-ing and ah-ing. Because he got the Tamiflu incredibly quickly, he got over the flu quickly.

"So the doctor was a week ahead of the results. If she had waited for the lab to come back, he could have had five or six horrible days of feeling ill.

"I think the Government has done the right thing. I can see why they wanted to do lab tests at the beginning, but it's out there now. The doctor had to come to us to take those swabs at 7.30[pm], putting herself in danger of infection. If she had had 20 cases, she would be taking swabs until 11.30 at night, and what's the point? By the time we get the results, he will be back at school."

40

People will die of swine flu in Britain every day by September if the current fatality rate remains the same and infection levels rise as expected.