By MIKE STOBBE, AP Medical Writer Mike Stobbe, Ap Medical Writer – Fri Jul 24, 11:16 am ET

ATLANTA – U.S. health officials say swine flu could strike up to 40 percent of Americans over the next two years and as many as several hundred thousand could die if a vaccine campaign and other measures aren't successful.

Those estimates from the Centers for Disease Control and Prevention mean about twice the number of people who usually get sick in a normal flu season would be struck by swine flu. Officials said those projections would drop if a new vaccine is ready and widely available, as U.S. officials expect.

The U.S. may have as many as 160 million doses of swine flu vaccine available sometime in October, and U.S. tests of the new vaccine are to start shortly, federal officials said this week.

The infection estimates are based on a flu pandemic from 1957, which killed nearly 70,000 in the United States but was not as severe as the infamous Spanish flu pandemic of 1918-19. But influenza is notoriously hard to predict. The number of deaths and illnesses would drop if the pandemic peters out or if efforts to slow its spread are successful, said CDC spokesman Tom Skinner.

A CDC official said the agency came up with the estimate last month, but it was first disclosed in an interview with The Associated Press.

"Hopefully, mitigation efforts will have a big impact on future cases," Skinner said.

In a normal flu season, about 36,000 people die from flu and its complications, according to American Medical Association estimates. Because so many more people are expected to catch the new flu, the number of deaths over two years could range from 90,000 to several hundred thousand, the CDC calculated. Again, that is if a new vaccine and other efforts fail.

The World Health Organization says as many as 2 billion people could become infected over the next two years — nearly one-third of the world population. The estimates look at potential impacts over a two-year period because past flu pandemics have occurred in waves over more than one year.

WHO officials believe the world is in the early phase of the new pandemic.

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 reported cases, according to numbers released Friday morning.

Because the swine flu virus is new, most people haven't developed an immunity against 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.

New swine flu illness have erupted through the summer, which is also unusual, though cases were less widespread this month. Officials fear an explosion of cases in the fall, when children return to school and the weather turns cold, making the virus easier to spread.