Dr Tini Garske explains her research to Fergus Walsh
The government must map the spread of swine flu more accurately in order to predict the number of people who are likely to die from it, scientists say.
Researchers at Imperial College say data is vital to ensure the country is "best prepared to fight the pandemic".
They predict that one in 200 people who get swine flu badly enough to need medical help could go on to die.
But the government's chief medical adviser said there was "no reason" to focus on establishing a single figure.
Meanwhile, the BBC understands that vaccines may not be ready until later than the government had predicted.
Medical correspondent Fergus Walsh said World Health Organisation officials expected the first stocks to be available in September or October, not August as ministers had said.
In any event, it will be the end of the year at least before there are sufficient quantities to immunise half of the UK population.
Chief medical officer Liam Donaldson also told the BBC that to cope with "the height of the pandemic", the government was considering changing the rules to speed up the death certification process for swine flu victims.
"We want to try and reduce as much as possible the burden of work on doctors and we are considering all sorts of things which will help will that," he said.
"That's one of the options that's being looked at."
Margin of error
Accurate predictions about the number of deaths likely to occur from swine flu are not yet available.
The health secretary says 100,000 a day could contract swine flu
Current estimates suggest it is about as virulent as some types of seasonal flu, but far less deadly than some previous flu pandemics.
Any estimates about swine flu are subject to a wide margin of error, not least because not everyone who catches it develops symptoms.
But despite the difficulties, the Imperial College scientists - who are advising the government on its swine flu strategy - say more accurate mapping of the spread of the virus must be carried out if it is to be effectively managed.
Their work is published in the British Medical Journal.
Dr Tini Garske said: "If we fail to get an accurate prediction of severity, we will not be providing healthcare planners, doctors and nurses, with the information that they need to ensure they are best prepared to fight the pandemic as we head into the flu season this autumn."
She said data must be collected "according to well designed study protocols and analysed in a more sophisticated way than is frequently being performed at present".
'Only an estimate'
Not everyone who is infected with swine flu will become ill enough to report their case to a doctor.
Of the proportion who do, scientists predict that 0.5% of them - one in 200 - could go on to die.
Health Secretary Andy Burnham has said in the worst case there could be 100,000 new cases of swine flu a day later in the year, although many of these may not fall seriously ill.
There need to be plans for hospitals to share workloads across areas
Prof Steve Field Royal College of GPs
The chief medical officer told BBC Radio Four's Today programme the underlying message of the Imperial report was that it was very difficult to make forecasts.
"If you look at statistical modelling, it's very valuable, but you do have to treat it with a lot of caution early on," Sir Liam said.
"We know that, for example, from the CJD epidemic where early predictions were of hundreds of thousands and millions of cases, when in fact there have been 164."
At present, he said, swine flu appeared to be less severe than previous pandemics and "broadly similar" to seasonal flu - which kills between 5,000 and 7,000 each year.
There have so far been 17 swine flu-related deaths in the UK.
On Tuesday, a post-mortem examination ruled that a GP who died after contracting it was not killed by the virus.
Professor Steve Field, from the Royal College of GPs, said plans to manage the outbreak were on schedule.
"What we're learning is this is happening in hotspots around the country... so there need to be plans for individual hospitals and for hospitals to share workloads across areas," he said.
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.
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