GENEVA — Up to 2 billion people could be infected by swine 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.
In Mexico, the hardest-hit country so far, high schools and universities opened for the first time in two weeks as Health Secretary Jose Angel Cordova insisted the epidemic is on the decline.
"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 a third of the world's population would get infected with this virus."
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With the current total population of more than 6 billion, that would mean an infection total of 2 billion, he said, but noted that the world has changed since pandemics of earlier generations, and experts are unable to predict if the impact will be greater or smaller.
Chris Smith, a 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 told The Associated Press. "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," Smith said.
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 swine 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."
People react differently to the flu depending on their general state of health and other factors, he said. Some younger people in the Southern Hemisphere may be more vulnerable because of poor diet, war, HIV infections and other factors.
"We expect this kind of event to unfold over weeks and months," Fukuda said. "Really if you look over a two-year period, that is really the period in which you see an increase in the number of illnesses and deaths during a pandemic influenza."
Pima County health chief cautious / A14
at a glance
Key developments on swine flu outbreaks, according to the U.S. Centers for Disease Control and Prevention, the World Health Organization and government officials:
• Confirmed cases: More than 2,350 worldwide in 26 countries, including more than 1,160 in Mexico, about 900 in the United States and about 200 in Canada.
• CDC says only about 10 percent of Americans with swine flu are believed to have gotten it during trips to Mexico. Over the weekend it said about a third of the U.S. cases were people who had been to Mexico, where the outbreak began.
• President Obama's 2010 budget would devote an additional $584 million to pandemic flu- preparation efforts. That's on top of $1.5 billion he requested from Congress for this year when the flu first emerged.
• China releases a group of people quarantined for a week after being on the same flight as a Mexican man diagnosed with swine flu.
The Associated Press
