Just Like Wine…
May 22, 2009
Older adults might have some pre-existing immunity to the 2009 H1N1 (swine) flu virus, according to the CDC.
Serological tests showed that some adults, particularly those over 60, had antibodies effective against the new strain, Jacqueline Katz, Ph.D., of the influenza division of the CDC’s National Center for Immunization and Respiratory Diseases, and colleagues wrote in the May 22 issue of Morbidity and Mortality Weekly Report.
But Anne Schuchat, M.D., interim deputy director for the CDC’s science and public health programs, 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, epidemiological evidence continues to point to a disproportionate number of infections in school-age children and younger adults.
Of the cases reported to the CDC, 64% are in 5- to 24-year-olds and just 1% are in individuals older than 65, an unusual pattern compared with seasonal influenza.
This has led to speculation that older individuals have at least some degree of pre-existing immunity, possibly from years of immunization with seasonal flu vaccines. They contained 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,” Dr. Schuchat said.
She 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.
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.
Seasonal vaccines from the past four flu seasons induced some cross-reactive antibody response to the new virus, but probably not enough to stave off infection.
“CDC does not believe that seasonal influenza vaccine would provide any meaningful protection against this novel H1N1 strain,” Dr. Schuchat said.
Agency researchers analyzed 359 stored serum samples from individuals ranging from 6 months to 64 years, before and after vaccination during flu seasons since 2005-2006. The samples were obtained during U.S. and European vaccine studies.
Both before and after vaccination, children up to 9 years had no level of cross-reactive antibody to the new virus.
Before vaccination, in up to 9% of adults ages 18 to 64, and in 33% of those older than 60, cross-reactive antibody to the new H1N1 virus was present at levels that have been associated with a 50% reduction in the risk of influenza infection or disease.
This pre-existing immunity possibly results from previous exposure to H1N1 viruses more similar to the new virus than to current seasonal H1N1 viruses — either through vaccination or infection.
After receiving a seasonal flu shot, up to 25% of 18- to 64-year-olds and 43% of individuals over 60 had cross-reactive antibody to the new virus.
Dr. Schuchat discounted the apparent booster effect following vaccination, noting that the results were based on a relatively small number of specimens and an unconventional test of antibody response to influenza.