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 are not 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 stricken by swine flu. Officials said those projections would drop if a new vaccine is ready and widely available, as U.S. officials expect.

Related: 10 Ways to Prevent Swine Flu

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.

First identified in April, swine flu has so far caused about 263 U.S. deaths, according to the latest numbers. CDC officials say it's likely that more than 1 million Americans have become sick from the virus, although many were probably mild cases never reported.

Because the swine flu virus is new, most people have not 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.