The Vaccine Safety Datalink (VSD) is a collaborative project involving CDC and eight health maintenance organizations (HMOs) in the United States. Computerized data on vaccination, medical outcomes, and patient demographics are collected and linked under a standard protocol at multiple HMOs. Beginning with the 2003-04 influenza season, the VSD team and one of the HMOs, Kaiser Permanente Northern California (KPNC), established an automated system for rapid detection of potentially adverse events after vaccinations among its members. During the 2004-05 influenza season, in response to the influenza vaccine shortfall and resulting prioritization of vaccine distribution, this rapid analysis system also was used to assess influenza vaccination coverage weekly among KPNC members. The results indicated that KPNC followed Advisory Committee on Immunization Practices (ACIP) prioritization guidelines by targeting influenza vaccination to children aged 6-23 months and adults aged >/=65 years. For the 2005-06 influenza season, the rapid analysis system should be expanded to include data from additional HMOs and more detailed information on vaccinees (e.g., high risk for influenza complications) to better characterize influenza vaccination coverage during the 2005-06 influenza season on a weekly basis.