ATLANTA - New test results show what scientists have suspected - people in their 60s and older have signs of greater immunity to the new swine flu virus.
Scientists think it's because older people have been exposed to other viruses in the past that are more similar to swine flu than more recent seasonal flus.
But the results come from complicated lab work and calculations, and it's not yet clear how safe older people actually are from the new infection, federal officials said.
"We can't say," said Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention. So older people are advised to take the same precautions as their children and grandchildren.
In the new analysis, CDC scientists mixed the new swine flu virus with blood samples taken in the past from people in the United States and Europe to check for antibodies that would guard against infection.
The samples were taken from healthy people before the new flu surfaced.
Significant protection was seen in one in three of the samples from people 60 and older, but in less than one in 10 of samples from younger adults.
The number of samples was small, and the lab test hasn't been verified, so cautious CDC scientists weren't willing to say it shows a clear-cut immunity in older people.
However, it could have implications for how well different people do when exposed to the swine flu virus. Also, if the government pushes two-dose 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.
Also yesterday, Mexico City lowered its swine flu alert level from yellow to green, and the mayor said "we can relax" because there had been no new infections for a week.
Mayor Marcelo Ebrard said the change means the risk of contagion is low, the situation is under control, and the images of countless people wearing blue surgical masks in cars, sidewalks, restaurants, and theaters can be consigned to history.