The mechanism by which the 2009 H1N1 influenza virus caused more severe disease manifestation in middle-aged adults than elderly or young children is reported in this week’s issue of Nature Medicine.
The age distribution of severe cases of disease during the 2009 H1N1 pandemic was unusual since influenza viruses typically affect those with weaker immune systems — such as elderly and young children. Fernando Polack and colleague found severe cases in middle-aged adults — between 17 and 57 years of age — were caused when pre-existing antibodies against seasonal strains of influenza cross-reacted with the H1N1 strain. These antibodies do not protect against H1N1 but are associated with immune complex-mediated disease, whereby an aggregate of antibodies and viral proteins accumulate in organs, such as the kidneys or lung, leading to post-infection illness.
The team compared lung samples from middle-aged patients who died from the H1N1 virus to fatal cases from the 1957 H2N2 influenza, and found that that the mechanisms of immune complex disease were similar in both cases.