GENEVA -- Up to 2 billion people could be infected by A (H1N1) flu if the current outbreak turns into a pandemic lasting two years, the World Health Organization said Thursday.
WHO flu chief Keiji Fukuda said the historical record of flu pandemics indicates one-third of the world's population gets infected in such outbreaks. Independent experts agreed that the estimate was possible but pointed out that many would not show any symptoms.
Students from Mexico's National Autonomous University, UNAM, some wearing face masks as a precaution against swine flu, return to classes in Mexico City, Thursday, May 7, 2009. [Agencies]
In Mexico, the hardest hit country so far, high schools and universities opened for the first time in two weeks as the government's top health official insisted the epidemic is on the decline. All students were checked for A(H1N1) flu symptoms and some were sent home.
"If we do move into a pandemic, then our expectation is that we will see a large number of people infected worldwide," Fukuda said. "If you look at past pandemics, it would be a reasonable estimate to say perhaps a third of the world's population would get infected with this virus."
With the current total population of more than 6 billion, that would mean an infection total of 2 billion, he said, but added that the world has changed since pandemics of earlier generations, and experts are unable to predict if the impact will be greater or smaller.
"We don't really know." said Fukuda. "This is a benchmark from the past. Please do not interpret this as a prediction for the future."
Chris Smith, at flu virologist at Cambridge University in England, said the 2 billion estimate was possible.
"That doesn't sound too outlandish to me for the simple reason that this is a very infectious virus," Smith said. "You're talking about a virus that no one in the population has seen before and therefore everyone is immunologically vulnerable. Therefore it's highly likely that once it starts to spread, people will catch it. And since the majority of the world's population are in contact with one another, you're going to get quite a lot of spread."
John Oxford, professor of virology at St. Bart's and Royal London Hospital, agreed.
"I don't think the 2 billion figure should scare people because it's not as though 2 billion people are going to die. The prediction from WHO is that 2 billion people might catch it. Half of those people won't show any symptoms. Or if they show any symptoms, they will be so mild they will hardly know they've had it."
Fukuda said it also is impossible to say if the current strain of the flu will become severe or mild, but that even with a mild flu, "from the global perspective there are still very large numbers of people who could develop pneumonia, require respirators, who could die."
A mild outbreak in wealthier countries can be "quite severe in its impact in the developing world," Fukuda said.