During the influenza A (H1N1) 2009 pandemic, the Countermeasure and Response Administration (CRA) system, a Centers for Disease Control and Prevention (CDC) computer-based informatics application, monitored H1N1 vaccine uptake during the early stages of the US vaccination program, from October through the end of November 2009. CRA, which directly monitors vaccine doses administered, was developed to support the mass tracking of medical countermeasure use during public health events and to complement population-based survey data on vaccination coverage during a pandemic influenza vaccination program. CRA provided weekly near real-time reports of H1N1 vaccine doses administered at national and state levels. On average, during any given week, 58.8 percent of the total data available to be reported was actually reported to CDC. During the 8-week mandatory reporting period, a cumulative total of 13,109,962 first-dose vaccine doses administered were reported through CRA, representing approximately 4.4 percent of the US population. Nearly 60 percent of these doses were administered to individuals aged 6 months to 24 years, an age interval that was included in the initial target groups prioritized to receive vaccine. CRA was a key component of the national surveillance system providing information on early uptake of H1N1 vaccine and monitoring program progress. These accomplishments indicate that CRA can effectively function as an immunization tool to monitor vaccine uptake during a pandemic.