CHICAGO Swine flu infected 14 times as many children as adults over 60 in Chicago, city health department officials reported on Thursday in one of the first detailed looks at the new pandemic virus.
No children have died, but the officials said their analysis suggests that prevention efforts should focus on children.
In many ways, the new H1N1 flu virus acted like the typical seasonal flu in Chicago, causing fever, cough and sore throats for most people, said Dr. Susan Gerber, chief medical officer at the Chicago Department of Public Health.
"What was different was that younger age groups seemed to be getting it more often than older age groups," said Gerber, who reported on the city's swine flu cases in the U.S. Centers for Disease Control and Prevention's weekly report on death and disease.
"Our median age for all of the cases reported to the Chicago Health Department was 12 years old. That's obviously a younger age," Gerber said in a telephone interview.
The account offers a snapshot of how one of the biggest U.S. cities dealt with the new H1N1 virus, which is still spreading globally and which experts fully expect to start causing more disease as the weather cools.
Separately on Thursday, California's top health official predicted that as many as one in four Californians could be affected by the swine flu this autumn.
"While none of us can predict the course of the novel H1N1 influenza outbreak, it has the potential to sicken millions of people in our state in the months ahead and as many as 1 in 4 Californians may be affected," said Dr. Mark Horton, director of the California Department of Health.
Swine flu has infected well over 1 million people in the United States, and is now the CDC's No. 1 priority. Other research also shows that older children and young adults are by far the most likely to become infected with the new virus.
The World Health Organization predicts a third of the world's population will eventually be infected. Seasonal flu infects between 5 percent and 20 percent of a given population every year, but 90 percent of severe cases and deaths are among the elderly.
This virus is different.
Researchers from the Chicago Health Department analyzed 1,557 confirmed cases of infection with the H1N1 virus between April and July and found it disproportionally targets younger people, confirming trends seen in other areas of the country.
Among the confirmed cases, the "attack rate" -- the rate of infection -- was highest among children aged 5 to 14, followed by children under 4. Hispanics, blacks and Asians and Pacific Islanders had higher reported rates than whites.
"We also found that hospitalizations were primarily in younger age groups, and that indeed, the median age was 16 years old. That is a younger age group," Gerber said.
So far, 205 people have been hospitalized with swine flu. Children 4 and under represent the biggest age group, followed by children 5 to 17. Forty people, or 20 percent of the hospitalized patients, were admitted to the intensive care unit, and nine needed a ventilator.
Most -- 117 -- had underlying health conditions, most commonly asthma and diabetes, and 14 women were pregnant. Of the 7 deaths reported so far, all but two were people who had underlying health problems.
The youngest person to die was a 20-year-old woman who died form respiratory failure a day after giving birth by an emergency caesarean section.
(Additional reporting by Dan Whitcomb in Los Angeles; editing by Maggie Fox and Mohammad Zargham)