Health Secretary Andy Burnham: 'The vast majority... have shown mild symptoms'
Tens of thousands of cases of swine flu could be emerging each week by the autumn, the chief medical officer says.
Sir Liam Donaldson said the most likely scenario was that the UK would see a surge of cases from the current levels.
Over 3,700 cases have been confirmed so far, and the government has already paved the way for some areas to move away from trying to contain the virus.
Glasgow, London and the West Midlands have been identified as the areas with the most rapid spread.
To date, local health officials have been attempting to put a ring around the outbreaks by tracing the people with whom confirmed cases have come into contact and giving them drugs to try to prevent flu developing.
We still think we are heading for a larger surge of cases in the autumn and winter
Sir Liam Donaldson, Chief Medical Officer
This approach has also involved the closure of several schools.
But Sir Liam said certain areas were seeing rapid community spread which meant this was not now practical.
In these areas, anti-viral drugs may in the future only be given to those people showing symptoms, while the tracing of contacts will stop.
However, he said this pattern was not emerging everywhere, with some parts of the UK - Wales and Northern Ireland in particular - hardly seeing any cases.
He said the contingency plans always envisaged the containment phase would not last indefinitely.
Sir Liam said: "We still think we are heading for a larger surge of cases in the autumn and winter.
"Although it still remains a fairly mild virus and in most places is only spreading gradually."
It also seems likely the UK will receive the first batches of vaccines slightly earlier than normal.
Sir Liam said it may be available from August rather than the autumn as first expected.
The government has contracts with two manufacturers for 132m doses - enough for the whole population.
Sir Liam acknowledged that they would not get all the jabs in one go, but added this did not necessarily matter.
"In all previous pandemics excess mortality has occurred for three years after the new virus emerges. It is not just a consideration for this winter."
Shadow Health Secretary Andrew Lansley said: "We are now moving into a new phase of more community transmission. While we have supported the strategy to date, the government needs to provide more clarity on some key issues."
In particular, he highlighted school closures, the distribution of anti-virals and the vaccination programme.
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