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 the government's top health official insisted the epidemic is on the decline. All students were checked for swine flu symptoms and some were sent home.
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."
So far the swine flu virus has spread to 26 countries. Brazil and Argentina on Thursday became the second and third countries in South America to announce confirmed cases.
WHO raised its global total of laboratory-confirmed cases to 2,099, from 1,893 late Wednesday, and said swine flu also has caused two deaths in the United States.
This swine flu seems to have a long incubation period — five to seven days before people notice symptoms, according to Dr. Marc-Alain Widdowson, a medical epidemiologist from the U.S. Centers for Disease Control and Prevention. That means the virus can keep being spread by people who won't know to stay home.