Tests Show More Swine Flu Immunity in Older Folks

However, it could have implications for how well different people do when exposed to the swine flu virus. Also, if the government pushes swine flu shots later this year, it might mean only one dose is necessary for senior citizens.

CDC officials say the Asian flu in 1957 -- which was a pandemic -- and some other flu viruses after, have become common seasonal flu strains. But scientists believe that before 1957, the kind of flu that infected the country each winter was more like the new swine flu, at least in the way patients' immune systems responded.

That would help explain why the new flu seems to be hitting younger people harder than older folks. Usually, the vast majority of flu-related hospitalizations are elderly people. But with the new virus, about 40 percent of those hospitalized have been in the 18 to 50 age group. For all cases -- not just those hospitalized -- more than 60 percent have been in people younger than 25.

But some people born before 1957 have still been getting sick from the swine flu, and health officials are urging older people to take the same precautions as everyone else.

More than 5,700 confirmed and probable cases have been reported in the United States. CDC officials say that not all people who get sick from the virus get tested, and it's possible that more than 100,000 Americans have had the infection. Worldwide, roughly 11,000 cases have been reported, with at least 85 deaths.

The CDC on Thursday said it knows of nine U.S. deaths associated with the outbreak, but news reports suggest there have been at least 10.

Swine flu cases seem to be decreasing in some parts of the country, based on visits to doctor's offices and hospital emergency rooms. The Southeast, Midwest and Southwest all have shown signs of decreased flu cases, Schuchat said.

But New England, New York and New Jersey seem to have more swine flu activity, she said.

"Just like weather, this is a local occurrence" that varies by region, said Schuchat.