ABSTRACT Investigators at Seattle Children's Hospital propose to establish a site to conduct population-based surveillance of children in King County (population 1.9 million) seeking emergency care or requiring hospitalization for vaccine-preventable or potentially vaccine-preventable conditions. The establishment of a New Vaccine Surveillance Network (NVSN) site in Seattle would provide West Coast representation within the national surveillance effort, capturing data from a part of the country with differences in both rotavirus seasonality and immunization rates for rotavirus vaccine. The Seattle investigators, led by Drs. Janet Englund and Eileen Klein, are experienced in surveillance for acute gastroenteritis, rotavirus disease and rotavirus vaccine-effectiveness studies, and have a history of successfully conducting collaborative innovative multicenter clinical studies of new and emerging pathogens, vaccines, and vaccine-preventable infections in diverse pediatric populations. Seattle Children's Hospital, the only pediatric medical center-emergency department in the county, encompasses 85% of pediatric hospital beds and cares for approximately 50% of children with acute gastroenteritis receiving inpatient and outpatient care in King County. Our state-wide immunization registry, which registers 100% of children born in-state and over 96% of immunizations given to young children, will assist in vaccine effectiveness studies. With collaboration from the Public Health Department of Seattle/King County (PHSKC), we propose to collect admission data from our hospital and utilize existing county-wide surveillance programs tracking all emergency room admissions and all hospitalizations to be able to calculate adjusted, populated-based disease and hospitalization rates. Our project objectives will be to establish a site for population-based surveillance by collecting epidemiologic and clinical data, specimens, and vaccination verifications from symptomatic and control subjects, and to evaluate rotavirus vaccine effectiveness. We will assess the impact of other new vaccines and new vaccine policies as appropriate, and work collaboratively with other sites and the CDC to understand the burden of vaccine-preventable or potentially vaccine- preventable diseases by assisting in developing and implementing new projects and new protocols. The Seattle site will contribute substantially to NVSN and their efforts in understanding and documenting the importance of specific pathogens and new vaccines.