The World Health Organization says as many as 2 billion people could become infected in the next two years — nearly a third of the world population. The estimates look at potential impacts in a two-year period because past flu pandemics have occurred in waves over more than one year.
Swine flu has been an escalating concern in Britain and some other European nations, where the virus' late arrival has grabbed attention and some officials at times have sounded alarmed.
In an interview Friday, the WHO's flu chief told the AP the global epidemic is still in its early stages.
"Even if we have hundreds of thousands of cases or a few millions of cases ... we're relatively early in the pandemic," Keiji Fukuda said at WHO headquarters in Geneva.
The first vaccines are expected in September and October, Fukuda said. Other vaccines won't be ready until well into the flu season when a further dramatic rise in swine flu cases is expected.
First identified in April, swine flu has likely infected more than 1 million Americans, the CDC believes, with many of those suffering mild cases never reported. There have been 302 deaths and nearly 44,000 laboratory-identified cases, according to numbers released Friday morning.
Because the swine flu virus is new, most people haven't developed an immunity to it. So far, most of those who have died from it in the United States have had other health problems, such as asthma.
The virus has caused an unusual number of serious illnesses in teens and young adults; seasonal flu usually is toughest on the elderly and very young children.