The UN health agency was fielding inquiries from the media and the public after Dr David Nabarro - a senior WHO official named on Thursday as the new UN coordinator for avian and human influenza - cited the number during a news conference at the UN's New York headquarters.
The WHO's flu spokesman at the agency's Geneva headquarters made a surprise appearance on Friday at the UN regular media briefing in an effort to put Nabarro's comments in context.
While he did not say the 150 million prediction was wrong or implausible, he reiterated that the WHO considers a maximum toll of 7.4 million a more reasoned forecast.
Scientists have made all sorts of predictions, ranging from less than 2 million to 360 million. Others have quoted 150 million. Last year, the 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.
"You could pick almost any number" until then, he said, adding that WHO "can't be dragged into further scaremongering."
Experts agree that there will be another flu pandemic - a new human flu strain that goes global.
"We're not going to know how lethal the next pandemic is going to be until the pandemic begins"
WHO influenza spokesman
However, it is unknown when or how bad that global epidemic will be.
It is also unknown whether the H5N1 bird flu strain circulating in Asian poultry now will be the origin of the next pandemic, but experts are tracking it in case, and governments across the world are preparing 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, or the proportion of the population that becomes infected. The other is the death rate, or the proportion of the sick who die.
Flu attack rates
Normal seasonal flu viruses have an attack rate of 5% to 20% but a death rate of less than 1%. Between 250,000 and 500,00 die from flu every year, according to the WHO.
Based on evidence from the three pandemics during the 20th century, scientists have determined that pandemic flu strains tend to infect 25% to 35% of the population.
The worst death rate was in the 1918 pandemic, known as the Spanish flu pandemic. That killed 2.6% of those who got sick, a total of 40 million people.
The effectiveness of anti-flu
drugs determines flu death rates
The 1957 pandemic killed 2 million people, and the most recent one, in 1968, killed 1 million.
Forecasts that change the assumed attack rate or the death rate will yield different predictions. Other assumptions, such as whether or not anti-flu drugs will work against the virus, also would change the figures.
The WHO said on Friday that it considered the most likely scenario to be a toll of 2 million to 7.4 million people.