American Journal of Epidemiology ª The Author 2011. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. Vol. 173, No. 10 DOI: 10.1093/aje/kwq497 Advance Access publication: March 22, 2011 Practice of Epidemiology Planning for the Control of Pandemic Influenza A (H1N1) in Los Angeles County and the United States Dennis L. Chao, Laura Matrajt, Nicole E. Basta, Jonathan D. Sugimoto, Brandon Dean, Dee Ann Bagwell, Brit Oiulfstad, M. Elizabeth Halloran, and Ira M. Longini Jr.* * Correspondence to Dr. Ira M. Longini Jr., Center for Statistical and Quantitative Infectious Diseases, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, M2-B865, Seattle, WA (e-mail: longini@scharp.org). Initially submitted June 16, 2010; accepted for publication December 15, 2010. Mathematical and computer models can provide guidance to public health officials by projecting the course of an epidemic and evaluating control measures. The authors built upon an existing collaboration between an academic research group and the Los Angeles County, California, Department of Public Health to plan for and respond to the first and subsequent years of pandemic influenza A (H1N1) circulation. The use of models allowed the authors to 1) project the timing and magnitude of the epidemic in Los Angeles County and the continental United States; 2) predict the effect of the influenza mass vaccination campaign that began in October 2009 on the spread of pandemic H1N1 in Los Angeles County and the continental United States; and 3) predict that a third wave of pandemic influenza in the winter or spring of 2010 was unlikely to occur. The close collaboration between modelers and public health officials during pandemic H1N1 spread in the fall of 2009 helped Los Angeles County officials develop a measured and appropriate response to the unfolding pandemic and establish reasonable goals for mitigation of pandemic H1N1. communicable disease control; influenza, human; influenza vaccines; mass vaccination Abbreviations: CDC, Centers for Disease Control and Prevention; DPH, Department of Public Health; FHCRC/UW, Fred Hutchinson Cancer Research Center/University of Washington; LA County, Los Angeles County. Pandemic influenza A (H1N1) emerged in Mexico in April 2009 and spread worldwide in less than a year. After causing intense but limited outbreaks, primarily among chil- dren, in the United States during the late spring of 2009, the virus caused sporadic outbreaks throughout the summer of 2009. Following the opening of US schools in the late sum- mer of 2009 (1), the epidemic peaked in early October among children, with limited spread of the infection to adults. Vaccination began in the United States in early Oc- tober 2009, and by December 2009 approximately 20% of the US population had been vaccinated with pandemic H1N1 vaccine (2). As of December 2009, an estimated 28% of the US population had been infected with pandemic influenza H1N1 (3). In the winter of 2007, an applied research collaboration between the Center for Statistical and Quantitative Infec- tious Diseases at the Fred Hutchinson Cancer Research Center/University of Washington (FHCRC/UW) (Seattle, Washington) and the Los Angeles County (LA County), California, Department of Public Health (DPH) was estab- lished with the goal of planning for an influenza pandemic using mathematical modeling. At the outset, the primary aim was to develop a simulation model for the county to assist the LA County DPH in planning for the mitigation and control of an avian influenza A (H5N1) pandemic should a reassorted or mutated virus demonstrate the capacity for widespread transmission between humans (4). As in other local health jurisdictions, planning for a pandemic without clear information about the timing, severity, extent, and du- ration of such an event was a challenge for the LA County DPH. The use of mathematical and computer models adapt- ed to local conditions provided the ability to conceptualize the potential spread and impact of the disease, as well as to quantify the expected effectiveness of the available 1121 Am J Epidemiol. 2011;173(10):1121–1130