U.S. health officials said the median age for confirmed hospital cases in the United States is 15. And in Mexico, new figures showed that almost half of the 42 confirmed swine flu deaths were of people 29 and younger.

Some experts have speculated that older people exposed to more flu strains in the past have built up greater immunity.

At a briefing on Wednesday, Dr. Richard Besser, acting director of the Centers for Disease Control, said older people's immunity might be one possible explanation. Or he said it might just be that younger people have tended to get sick first in the outbreak.

During the regular winter flu season, elderly people and those with chronic health conditions are the most likely to be sent to a hospital.

The age range of those in U.S. hospitals with swine flu is eight months to 53 years. There are 35 people hospitalized with confirmed cases of the new flu in 14 states, the CDC says.

Besser said the CDC has little information on other medical conditions of hospitalized patients that might have made them more vulnerable. So far, no single health problem has emerged as a dominant factor, he said.

On Tuesday, Mexico's Health Department released figures showing Mexicans 19 and younger accounted for more than half of those confirmed ill from the virus.