Health officials in Wisconsin, California, Illinois and Utah reported deaths from swine flu on Thursday and said all four patients had had other health problems.
The Wisconsin death, of a Milwaukee adult, was the state's first from the H1N1 virus. City Health Commissioner Bevan Baker would not release any details except to say that the person had a common underlying health condition that he would not specify.
A 74-year-old man from Gurnee, Ill., died Tuesday, according to the Lake County Health Department. Officials said he had significant medical conditions that increased his vulnerability.
Officials in California said a 9-year-old Concord girl had been diagnosed with swine flu and had a bacterial infection before she died May 29. The patient who died in Utah also was under 18, according to Gary Edwards, executive director of the Salt Lake Valley Health Department.
Officials with the U.S. Centers for Disease Control and Prevention said Thursday that there have been 11,468 probable and confirmed swine flu cases in the U.S., including 770 hospitalizations. They've confirmed 19 deaths; the four announced Thursday are not yet included in the government's tally.
The swine flu epidemic peaked in Mexico, the center of the outbreak, in late April, and now has spread throughout the Northern Hemisphere. 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.
On Thursday, Mexico had confirmed 5,717 cases, including 106 deaths, as scientists test a backlog of samples from patients.
Swine flu has hit more than 66 countries, with the United States reporting the most cases.
The CDC 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 report 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.