Researchers trying to find out why the incidence of H1N1 infection is lower than expected in older adults discovered that the current H1N1 flu strain is significantly different from H1N1 strains observed over the past few years as well as the normal seasonal flu virus. They add that this is especially true for epitopes, which are structural parts of the virus normally recognized by the immune system if the individual has had exposure through past infection or vaccination.
They hypothesize that the flu strains before 1957 may have had epitopes similar to the current H1N1 strain, so older adults may be somewhat protected. “Those born more recently have virtually no pre-existing immunity to this pandemic H1N1 strain because they have never been exposed to anything like it,” explains Richard Scheuermann, Ph.D., professor of pathology and clinical sciences at UT Southwestern and a co-author of the paper that appears in the Proceedings of the National Academy of Sciences.
“Normally, older adults are generally more susceptible to pathogens like influenza,” he continues. “However, for the pandemic H1N1 strain this does not seem to be the case. We speculate that older adults may have been exposed to viruses in their youth in which the epitopes are more similar.” At this point, he notes, scientists must continue to be vigilant about tracking the pandemic H1N1 strain as it continues to evolve.
The investigators examined whether epitopes present in the seasonal flu strains between 1988 and 2008 were also found in the existing H1N1 strain. They used information catalogued in databases of epitopes and influenza genetic sequences. The scientists found major genetic differences between the pandemic H1N1 strain and seasonal strains.