This novel, swine-origin, influenza A virus has now become the first pandemic of the twenty-first century. The international scientific, public health, security, and policy communities quickly mobilized to characterize the novel virus (hereinafter 2009-H1N1 influenza A) and address its potential effects. Within six months of the discovery of the 2009-H1N1 influenza A virus, researchers had gained considerable knowledge about the latest pandemic influenza virus and produced a vaccine against it, but many scientific and policy questions raised by the 2009-H1N1 influenza A virus remained to be answered.

The arrival of an influenza pandemic in 2009 was both anticipated and unexpected. That a novel, readily transmissible, influenza virus would spread widely and rapidly along with its globe-trotting hosts seemed inevitable; that this pandemic strain emerged in the Americas, rather than Asia, surprised many infectious disease experts. “We have all been preparing for a pandemic,” veteran flu researcher Robert Webster of St. Jude Children’s Research Hospital remarked recently (Webster, 2009). “H5N1 [Avian influenza] has been at the top of our list and surprise, surprise, 2009-H1N1 influenza A came out of left field.”

In the months since the initial identification of the 2009-H1N1 influenza A virus, the disease has now spread to over 213 countries and territories while scientists, healthcare providers, policy makers, the media, and the general public attempted to anticipate, and mitigate, the myriad potential consequences of the evolving pandemic. Studies of the evolution of influenza viruses attest to their essential unpredictability, but knowledge gathered during the recent influenza season in the Southern Hemisphere can inform strategies to address the expected resurgence of 2009-H1N1 influenza A with winter’s return to the Northern Hemisphere. This effort will also be advanced by the ongoing evaluations of public health capacities to address current and future challenges presented by this pandemic, its economic repercussions, and its sociopolitical effects.

In the recent past, the Institute of Medicine’s (IOM’s) Forum on Microbial Threats has convened several workshops focused on pandemic disease emergence, spread, and response. The first followed the emergence of severe acute respiratory syndrome (SARS) in 2003 (IOM, 2004); others considered the potential global threat posed by H5N1 influenza virus (IOM, 2004, 2005, 2007a,b) and the dynamics of infectious disease transmission in a highly interconnected world (IOM, 2010). Within months of its declaration as the first pandemic of the twenty-first century, the Forum convened a 2-day public workshop, on September 15th and 16th, 2009, to discuss the domestic and international impacts of, and responses to, the 2009-H1N1 influenza A pandemic. Through invited presentations and discussions, participants explored the origins, evolution, and epidemiology of the 2009-H1N1 influenza A virus; potential lessons learned from 2009-H1N1 influenza A infection patterns in the Southern Hemisphere; the role of disease detection, surveillance, and reporting in mapping and anticipating disease spread and evaluating the effects of mitigation measures; progress toward and prospects for vaccine and drug development and availability; considerations