More than 80% of children under the age of five who have been hospitalised with swine flu had no underlying health problems, according to a Department of Health patient survey.

Almost half of those aged 16-44 who needed inpatient treatment for the infection had also been previously healthy, it was revealed today.

The figures underline the vulnerability of the very young and the dangers posed by the H1N1 virus. The random sample of 192 patients showed that more than one in four patients admitted to hospital had asthma, more than 15% had heart disease and more than 10% had diabetes.

The survey was released as the Health Protection Agency reported 14,000 new cases of swine flu in England in the past week, up almost half on the previous week. The rate of increase, however, has slowed marginally.

The outbreak appears to be more virulent in the north of England. A total of 286 people are being treated in hospital, up from 218 last week. The number of people in intensive care has also risen to 36 from 25 in the previous week.

Two more swine flu related deaths occurred last week in England, bringing the UK total to 84, with 72 in England, nine in Scotland, two in Northern Ireland and one in Wales.

Since the start of term, 79 schools have suffered swine flu outbreaks, including 39 in Yorkshire and the Humber, 12 in the West Midlands and 12 in London.

"There is a steady rate of increase, but there is no sign of the explosive surge you can see in a pandemic," the chief medical officer, Sir Liam Donaldson, said. "There is still time for that to happen."

The government hopes to begin vaccinating high-risk groups and frontline health workers against swine flu in the second half of October, depending on when batches of a GlaxoSmithKline vaccine are delivered. Only children who fall within high risk groups – such as those with asthma or diabetes – are currently prioritised to receive the vaccine.

The number of beds in a specialist unit in Lecister for treating people with severe lung failure will be doubled from five to 10. The centre operates a procedure called extra corporeal membrane oxygenation (Ecmo) where blood is circulated outside the body and oxygen added artificially.

The Conservative health spokesman, Andrew Lansley, said: "As the government begins to roll out the swine flu vaccination programme for priority groups, the public needs to know [if] the government's advisory committee on vaccines will also recommend [on] whether it would be beneficial for the wider population to be given the vaccine as well."