Vaccine should be available for half the population by the end of the year
A third of the population may catch swine flu this winter and the virus could be here for up to five years, the government's medical chief has warned.
But Sir Liam Donaldson told the BBC the deaths of people with no apparent health problems did not mean the virus was becoming more severe.
A fuller profile of the disease was emerging, he said, in which the majority were emerging unscathed.
Nearly 200,000 concerned people have contacted NHS Direct since April.
On Monday, the NHS recorded the highest number of calls yet as news of two deaths of people with the swine flu virus broke.
A post-mortem has established that GP Dr Michael Day died of causes other than swine flu, while tests are still being carried out to establish why six-year-old Chloe Buckley died.
Seventeen people with swine flu have died in the UK so far.
Death figures 'premature'
Sir Liam, who is chief medical officer for England, said it was "too early to say" whether a mortality rate of one in 200 - as suggested by some experts - was accurate.
"We just haven't seen enough cases in this country to give an accurate figure.
"We're tracking the pandemic very closely and scientists are looking at the virus to see is there are any signs of mutation - we haven't seen anything of that.
"It's still an emerging profile so putting a figure on the death rate is premature.
"But the longer it goes on - as the virus passes through more and more people - you do have to be alert to it changing. But provided it stays within the present strain then it will probably produce a similar profile of illness."
Between 30 to 35% of people could come down with the virus this winter, he said. The disease was also likely to strike again in the years that followed.
"The virus will not just be here for one winter - previous pandemics have been around three, four, five years," Sir Liam said.
But by the end of the year, there should be enough vaccine in the UK to immunise half the population, he said.
It is still unclear who will get priority when a first batch comes through in late August, but it is thought likely to be those with underlying health problems and frontline health workers.