DoH officials have a list of key concerns, CMO Sir Liam Donaldson revealed last week. 'The biggest worry of all is the fear of mutation,' he said.

Sir Liam said he had been made aware of potential changes to the virus in Norway or France: 'Those have been investigated and there is no sign of any change in the virus. But it always remains a worry.'

Professor David Salisbury, DoH director for immunisation, said more research was needed on whether the swine flu vaccine would protect for more than one season against a drifted strain.

But he predicted that the H1N1 swine flu strain would be among those included in next year's seasonal flu jab.

Professor Salisbury said the virus's activity could vary widely. 'Everybody is being very cautious about 2010, because we simply have no way of knowing what will happen to this virus as it transitions from its current pandemic state to a seasonal virus over next year.'

He said that expanding population immunity may force the virus to drift. Sir Liam said there were also concerns about the virus's activity this winter.

'We remain worried that we might have a bad winter with other viruses coming in, with normal seasonal flu possibly affecting the elderly, and extreme cold weather,' he said.

The DoH also remained concerned about the 'continuing flow' of children aged under five into hospital, he said.

Swine flu cases in England halved last week, with a 'substantial' decrease in the estimated number of cases in England, from 46,000 to 22,000.

There was also a small decrease in GP consultation rates, down to 38.6 per 100,000 from 39.2 the previous week.

Meanwhile, the updated BMA and RCGP pandemic flu guidance now focuses specifically on the threat of swine flu.

Dr Peter Holden, GPC negotiator and the BMA's pandemic flu lead, said: 'H1N1 has behaved very differently to the avian flu that we had originally planned for.

'There is a huge amount of information out there for practices and this brings it all together.'