Influenza pandemic warning signals: Philadelphia in 1918 and 1977–1978

Global public health has struggled to provide timely warning of influenza pandemics. In this study, we review the signal pattern of local media reporting associated with the 1918 type A/H1N1 influenza pandemic and subsequent return of the A/H1N1 virus in 1977 and 1978 in Philadelphia. Open source local media reports are a critical source of warning intelligence for influenza pandemics. Documentation and analysis of pandemic influenza signal patterns is essential to capture lessons in effective warning intelligence for health security.