June 21, 2010 Hong Kong researchers suggest a new theory for why
swine flu infections turned out to be so mild. Prior exposure to
seasonal influenza A, either infection or vaccination, may
induce a cross-reactive immune response against the pandemic
virus. They report their findings in the July 2010 issue of the
Journal of Virology. Although the outbreak of human H1N1 in 2009
spread to pandemic proportions, the illness was considered mild
in most patients compared to seasonal influenza. Currently
available seasonal flu vaccines do not offer cross-reactivity to
pandemic H1N1 in any age group, suggesting that individuals
previously infected or exposed to seasonal influenza A viruses
may have memory cell-induced cross-protection to pandemic H1N1.
Prior research showed humans having cross-reactive memory cells
to a wide range of H5N1 peptides despite any previous exposure
to avian influenza A (H5N1). In this study researchers
determined that memory cells established by seasonal influenza
viruses can break down pa