Prior Exposure to Seasonal Influenza May Explain the Mildness of the 2009 H1N1 Pandemic

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 pandemic H1N1-infected target cells and ultimately induce cross-protective antibodies.

"Our data suggest that individuals who were infected with seasonal human influenza A viruses previously or who received seasonal human influenza vaccines may derive benefit, at least in part, from the preexisting cross-reactive memory cytotoxic T lymphocytes to reduce the severity of pdmH1N1 infection even without protective antibodies," say the researchers.

Reference: W. Tu, H. Mao, J. Zheng, Y. Liu, S.S. Chiu, G. Qin, P.L. Chan, K.T. Lam, J. Guan, L. Zhang, Y. Guan, K.Y. Yuen, J.S. Malik Peiris, Y.L. Lau. 2010. Cytotoxic T lymphocytes established by seasonal human influenza cross-react against 2009 pandemic H1N1 influenza virus. Journal of Virology, 84. 13: 6527-6535.