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.
Swine Flu: Questions Persist About Novel Strain
Schuchat noted, however, that genetic testing has found that the new H1N1 virus is not a close relative to any of the other H1N1 viruses that have circulated among people.
Other flu experts speculated that older patients might start getting infected in higher numbers as the outbreak progresses.
With seasonal influenza, children are the first to become infected, with infections picking up in older adults later in the season.
Dr. Marvin Bittner of Creighton University School of Medicine in Omaha, said that phenomenon could be what's happening with the current outbreak.
"Possibly, it is just a matter of time before they see this influenza in older people," he said.
CDC researchers analyzed 359 stored blood samples from individuals ranging in age from 6 months to 64 years before and after vaccination during the four flu seasons since 2005-2006. The samples were collected during U.S. and European vaccine studies.
The results indicated that both before and after vaccination, children up to 9 years old had no level of protection against the new virus.
On the other hand, up to 9 percent of adults ages 18 to 64 and 33 percent of those older than 60 had evidence of protection against the new virus before receiving a seasonal flu shot.
Schuchat said the findings should be "taken with caution" because of the relatively small number of blood samples and the use of an unconventional test of immune response to flu.
The tests also confirmed that seasonal flu vaccine is likely to have "little or no immune benefit" in protecting against the new virus, as many researchers had suspected.