In this file photo, a nurse administers a flu vaccine to an older patient while others wait in line... 
(Getty Images)Older adults might have some pre-existing immunity to H1N1 swine flu, according to the U.S. Centers for Disease Control and Prevention.
Lab tests showed that some adults, particularly those older than 60, had antibodies against the new strain, but Dr. Anne Schuchat of the CDC cautioned against reading too much into the finding.
"We don't know yet what that will mean in terms of actual immunity or clinical protection," she said on a conference call with reporters.
As the worldwide H1N1 flu outbreak progresses, evidence continues to point to a disproportionate number of infections in school-age children and younger adults.
Of the swine flu cases reported to the CDC, 64 percent are in 5- to 24-year-olds and just 1 percent are in individuals older than 65. That's an unusual pattern compared with seasonal influenza, which primarily affects the very young or old.
This has led to speculation that older individuals have at least some degree of pre-existing immunity to swine flu, possibly from years of immunization with seasonal flu vaccines, which contain different H1N1 viruses than the current outbreak strain, or previous infection.
"The study we're reporting today provides a little clue that's consistent with that clinical observation," Schuchat said.
Dr. Donald Henderson, an infectious disease expert at the University of Pittsburgh, called the observation of comparatively few swine flu cases among older adults "at least provisionally reassuring."
The H1N1 virus responsible for the 1918 flu pandemic continued to circulate in the population until 1957, when an H2N2 virus displaced it, he said.
"Thus, the first experience with influenza for most individuals born between 1918 and 1957 would have been with H1N1," he said. Those people are now between 52 and 91 years old.
If that previous exposure provided residual protection against the new swine flu virus, as observations seem to support, "there may well be some degree of protection sufficient, at least, to prevent serious illness in a significant proportion" of the population, he said.
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