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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