PARIS (AFP) – Sketchy or distorted data could cause misleading early judgements about the threat posed by swine flu, experts writing in the British Medical Journal (BMJ) said on Tuesday.

According to the UN’s World Health Organisation (WHO), 94,512 cases of A(H1N1) influenza have been reported, causing 429 deaths.

These suggest a case fatality ratio — the proportion of deaths in the number of people known to have fallen sick — of around 0.5 percent, which is in the upper range for run-of-the-mill seasonal flu.

Imperial College London epidemiologists caution against “simple interpretations of these crude figures” to those grasping for a yardstick of the virus’s severity.

While not saying that the novel flu is any less — or any more — virulent than thought, the researchers point to “biases” in data collection.

It could be some time before the world gets a more accurate fix on the viral threat, they say.

One problem they note is the data trawl does not include people who catch the virus but who do not fall sick, or those who feel only a little under the weather and so do not bother to consult a doctor.

As a result, many more people may have been infected by the virus than is known, which brings its lethality index down.

Another curiosity is a large discrepancy in fatalities reported between countries. Mexico has the highest death ratio, with 119 out of 10,292 cases, which is more than twice the average reported in Canada, the United States and Europe.

One theorised explanation for this is that a nastier form of the virus could be circulating in Mexico, say the researchers.

But another could be that Mexican doctor are simply focussing on the severest cases, which means that the true number of infections is higher.

If so, swine flu’s case fatality ratio could be much lower than thought and comparable to ordinary flu.

On the other hand, there is likely to be a rise in the number of fatal heart attacks and strokes that are linked to influenza, and these may go largely unreported, says the paper.

Looking at future surveillance of the pandemic, the authors plead with health authorities to speed up reporting data.

One area of worry is the time gap between the onset of flu infection and knowledge of the outcome.

In other words, a watchdog is notified at first that someone has fallen sick with the disease but only later will it be told whether the patient has died or survived.

The delay can have important repercussions for managing the pandemic.

One worry is that the present strain of swine flu will mutate, picking up genes from ordinary viruses that could make it more virulent as well as contagious.

Yet the added virulence factor will only show up when data reveal that the case fatality ratio has suddenly ratcheted higher. That key piece of evidence could be masked if there is a long delay in reporting patients’ outcome.

“Given the expectation that antigenic drift or viral reassortment with co-circulating seasonal influenzas may well change the severity of the new influenza virus over the coming months, it is especially important that these biases are minimised,” the paper warns.

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‘Better data needed’ on swine flu

출처 : BBC 08:57 GMT, Wednesday, 15 July 2009 09:57 UK
http://news.bbc.co.uk/2/hi/health/8150952.stm

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.

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 rediction 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.

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.