We have demonstrated that Google Flu Trends performs less well when estimating surveillance data for laboratory-confirmed influenza, which is not surprising, as the Google Flu Trends algorithm was developed using only ILI data. There are several US influenza surveillance systems, and taken as a whole, they provide an excellent overview of influenza activity at any period during the influenza season. However, only CDC Virus Surveillance data tracks nationwide activity of laboratory-confirmed influenza. The original publication describing and validating the Google Flu Trends methods intentionally excluded specifics concerning the statistical model used out of concern that public knowledge of the search terms could alter its usefulness to track influenza activity. Nevertheless, without the publication of the Google Flu Trends statistical model, further independent, Methylhexanamine hydrochloride prospective validation or improvements upon the model are not possible. This study is subject to limitations. While US Influenza Virologic Surveillance System provides the best data source for following trends in laboratory-confirmed influenza infections, it is nevertheless a convenience sample of specimens sent to participating laboratories. In addition, health care seeking behavior, physician testing practices, and internet search behavior may change over time or through the course of an influenza epidemic, limiting the interpretation of correlation data from this analysis. In NPS2143 hydrochloride conclusion, Google Flu Trends may make a useful contribution to public health given the timeliness of the data and its close association with traditional US ILI surveillance system data. However, CDC ILI Surveillance and positive influenza tests were more correlated during the five years of this study, including the unusual 2003�C04 influenza season, than were Google Flu Trends and positive influenza tests. We hypothesize that differences in internet search behavior, patient health care seeking behavior, and physician testing practices may alter the correlation between influenza surveillance systems.