WHY did the 2009 swine flu pandemic kill so many more young adults than children? Paradoxically, it might be because of past exposure to seasonal flu.
When Fernando Polack of Vanderbilt University in Nashville, Tennessee, and colleagues studied 75 adults with swine flu they found severe cases had more antibodies that bound to the virus but didn’t kill it (Nature Medicine, DOI: 10.1038/nm.2262). A tangle of virus and antibodies in their lungs activated an immune system component called complement, which failed to clear the mess and instead attacked lung tissue.
Polack says adults acquire the weak antibodies from past bouts of flu, and that they bind to the novel virus just strongly enough to make it worse. The effect could pose problems for a universal flu vaccine, as it might elicit antibodies that do not bind strongly enough to every flu virus to kill it.
The source for this information is a study published in Nature entitled “Severe pandemic 2009 H1N1 influenza disease due to pathogenic immune complexes. It was published 5 Dec, and its abstract reads …
Pandemic influenza viruses often cause severe disease in middle-aged adults without preexisting comorbidities. The mechanism of illness associated with severe disease in this age group is not well understood. Here we find preexisting serum antibodies that cross-react with, but do not protect against, 2009 H1N1 influenza virus in middle-aged adults. Nonprotective antibody is associated with immune complex–mediated disease after infection. We detected high titers of serum antibody of low avidity for H1-2009 antigen, and low-avidity pulmonary immune complexes against the same protein, in severely ill individuals. Moreover, C4d deposition—a marker of complement activation mediated by immune complexes—was present in lung sections of fatal cases. Archived lung sections from middle-aged adults with confirmed fatal influenza 1957 H2N2 infection revealed a similar mechanism of illness. These observations provide a previously unknown biological mechanism for the unusual age distribution of severe cases during influenza pandemics.