The virtues of antigenic sin: consequences of pandemic recycling on influenza-associated mortality

The 1968 influenza A(H3N2) ‘‘Hong Kong’’ pandemic in the United States was characterized by recycling of the H3 antigen, which reemerged after 77 years of absence. Seroarchaeological studies conducted on blood samples, collected in early 1968, demonstrated that the majority of the very elderly had H3 antibodies prior to the time they were exposed to the 1968 A(H3N2) pandemic virus. These antibodies were remnants of exposure to H3N2 viruses circulating before 1891. Others have hypothesized that these pre-existing antibodies should have conveyed a protective effect during the 1968 A(H3N2) pandemic, but this has never been demonstrated. In this study, we used national pneumonia and influenza (P&I) mortality data to demonstrate that the elderly age cohort over the age of 77 was, in fact, protected from influenza-related mortality during the 1968 pandemic. We found that the risk of influenza-related mortality among the elderly aged 75 – 79 and older, measured as excess P&I mortality, did not increase during the pandemic relative to surrounding severe non-pandemic influenza seasons. These findings have an implication for pandemic planning and the prioritization of high-risk groups for vaccination in the scenario of vaccine shortage. D 2004 Elsevier B.V. All rights reserved.