The World Health Organization moved Friday to revise alarming predictions that a pandemic stemming from the bird flu virus ravaging parts of Asia could kill as many as 150 million people.
The U.N. health agency was deluged with inquiries after Dr. David Nabarro — named Thursday as the U.N. coordinator for avian and human influenza — cited the number during a news conference at the U.N.'s New York headquarters.
While WHO's flu spokesman at the agency's Geneva headquarters did not say the 150 million prediction was wrong, he emphasized that 7.4 million deaths is a more realistic estimate.
Scientists have made predictions ranging from less than 2 million to 360 million. Last year, WHO's chief for the Asia-Pacific region predicted 100 million deaths, but until now that was the highest figure publicly mentioned by a WHO official.
"We're not going to know how lethal the next pandemic is going to be until the pandemic begins," WHO influenza spokesman Dick Thompson said Friday.
"You could pick almost any number" until then, he said, adding that WHO "can't be dragged into further scaremongering."
Experts agree there will certainly be another flu pandemic — a new human flu strain that goes global. However, it is unknown when or how bad that global epidemic will be.
It also is unknown whether the H5N1 bird flu (search) strain circulating in Asian poultry now will be the origin of the next pandemic. But experts are tracking it just in case, and governments across the world are preparing themselves for such a possibility.
Two factors will have a major influence on how many people will die from the next flu pandemic, experts say. One is the attack rate — the proportion of the population that become infected. The other is the death rate, or the proportion of the sick who die.
Normal seasonal flu viruses have an attack rate of between 5 percent and 20 percent, but a death rate of less than 1 percent. Between 250,000 and 500,00 die from flu every year, according to WHO.
Based on evidence from the three pandemics that occurred during the 20th century, scientists have determined that pandemic flu strains tend to infect between 25 percent and 35 percent of the population.
The worst death rate was seen in the 1918 Spanish flu pandemic. That killed 2.6 percent of those who got sick, or about 40 million people.
The other two pandemics were gentler. The 1957 one killed 2 million and the most recent, in 1968, killed 1 million.
Forecasts that change the assumed attack rate or death rate will yield different predictions. Other assumptions, such as whether anti-flu drugs will work against the virus, also would change the figures.
WHO said Friday it considers the most likely scenario to be a death toll of between 2 million and 7.4 million people.
"That's because we looked at what happened in the 1918 pandemic. That caused the greatest number of deaths ever recorded from an infectious disease in a single year, by far. More than the black plague, more than any other infectious disease," Thompson said.
"That was an extreme of an already rare event. Pandemics normally have more modest death rates."
When the U.N. health agency advises countries on how to prepare for the next pandemic, it uses the most moderate scenario likely to happen. It does not mean that a more exaggerated scenario cannot happen, only that it is less likely, Thompson said.
"There is a limited amount of public health money available to countries and we have to give them the best guidance on how to spend that money," he said.
The H5N1 strain of bird flu has swept through poultry populations in large swaths of Asia since 2003, jumping to humans and killing at least 65 people — more than 40 of them in Vietnam — and resulting in the deaths of tens of millions of birds.
Most human cases have been linked to contact with sick birds. But WHO has warned the virus could mutate into a form that spreads easily among humans — changing it from a bird virus to a human pandemic flu strain.