But it will continue to be a threat south of the equator, where countries are entering the winter months and traditional flu season, according to the CDC study, one of the most comprehensive yet on the effect of the virus on people.
South America already has had more than 600 cases, including one death in Chile, while Australia has reported more than 500.
Swine flu has hit more than 60 countries, with the United States reporting the most cases -- more than 10,000, including at least 18 deaths, according to the World Health Organization and individual country reports.
As of May 29, Mexico had confirmed 5,337 cases, including 97 deaths, according to the CDC. Mexico's Health Department on Tuesday said the nation's swine flu death toll had risen to 103 as scientists test a backlog of samples from patients.
The CDC study said children and adults under 60 are at greater risk of dying, judging from confirmed cases. One reason could be that younger people and children haven't built up immunities to seasonal flu as older people have. About one-third of U.S. adults aged 60 and older who were tested had antibodies from vaccines or exposure to other flu strains that could also keep them from contracting swine flu, the study said.
In Mexico, only 2 percent of confirmed cases have been 60 years old or older. But 42 percent of patients were under the age of 15 and 32 percent were between the ages of 15 and 29. The remaining 24 percent were aged 30-59.
A huge backlog of suspected cases has made long-term predictions for the epidemic difficult, the CDC said, but "data suggest the outbreak likely has moved beyond its peak nationally" in Mexico.
Mexico, like the United States, has struggled to keep up with laboratory testing to confirm suspected cases of the flu. The outbreak has led to a surge in testing at Mexico's National Laboratory from 30 specimens to 900 daily.
The CDC has praised Mexico for its response. Mexico ordered schools closed April 27 and then followed up with a five-day national shutdown of nonessential businesses to curb the spread of swine flu.
"I think in retrospect some people might look back and say well maybe that was extreme. But from the public health perspective, we would say in the face of uncertainty that's erring on the side of being safe," said Dr. Scott F. Dowell, who heads the CDC's international swine flu team.
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