The UN health agency warned on Friday against "scaremongering" over an expected global flu pandemic, a day after one of its officials said it could claim 150 million lives.
Dick Thompson, spokesman for the World Health Organization's (WHO) anti-flu operations, said it is nearly impossible to predict the death toll of a global crisis that experts fear could be fueled by Asia's bird flu outbreak.
People should be wary of any figure they hear, because all are based on "guesswork," he said.
"We can't be dragged into further scaremongering," Thompson said.
His comments came a day after David Nabarro, the newly-appointed UN coordinator for avian and human influenza, said a pandemic could kill "anything between five and 150 million."
That was highest figure cited by a UN official as the world body steps up its campaign to get governments to brace for a pandemic, which experts say is long overdue.
"What David Nabarro was talking about was a range of expert opinions," Thompson said.
"You could pick almost any number -- there's this vast range -- and any of these numbers could be right."
The WHO's official estimate is that 2 million to 7.4 million people could die, said Thompson.
But that is just a guideline figure, he said.
It is provided primarily to help international preparations, such as drug stockpiling and quarantine plans, intended to lessen the impact once a pandemic emerges.
Seasonal epidemics of flu in different regions of the world kill an estimated 250,000 to 500,000 people a year, according to the WHO.
But global pandemics, caused by new variants of the virus, claimed millions of lives in the last century.
Such outbreaks occur roughly every 40 years, and the last was in 1968, when the estimated toll was up to 4 million, Thompson said.
Fears about the risk of a deadlier and far more infectious type of flu in humans have been growing because of the spread of the H5N1 strain of bird flu in poultry and wildfowl in Asia and parts of the ex-Soviet Union.
The H5N1 variant has been known to scientists for decades as a latent killer within the world's bird populations.
However, it became a global concern when in Hong Kong in 1997 it first mutated into a form lethal to humans, killing six people.
It resurfaced in 2003 in poultry flocks across Asia, and has since caused more than 60 human deaths. Millions of chickens and ducks were culled in efforts to halt the disease's spread.
Although human-to-human transmission has been limited, experts fear that H5N1 could combine with common human flu viruses that spread easily.
Predicting exactly how to fight such a mutant strain is a tall order, Thompson said.
"We won't know until this virus emerges," he said.
"There's no pandemic virus at the moment, so vaccine companies are in a bind" as they try to develop a treatment for a hypothetical strain.
The existing anti-viral drug Tamiflu is seen an effective way of slowing down a pandemic strain when it arises -- provided cases are discovered early and intervention is swift.
The WHO is stockpiling supplies of Tamiflu to help cash-strapped developing countries deal with an outbreak and stop further spread, after a donation from its Swiss manufacturer, Roche.
More than 30 governments have also ordered millions of doses from the pharmaceutical giant.
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