But Google Flu Trends doesn't replace the CDC's national surveillance program, Yood stresses. Although it matches CDC-detected trends to within 95 percent, it's less accurate at estimating actual rates of laboratory-confirmed flu, according to a study presented May 17 at the American Thoracic Society 2010 International Conference in New Orleans.
Focusing on flu outbreaks in the U.S. between 2003 and 2008, Justin Ortiz from the Division of Pulmonary and Critical Care Medicine at the University of Washington and colleagues from CDC and PATH, a Seattle-based nonprofit health organization, found that Google Flu Trends deviated greatest from CDC surveillance figures for laboratory-confirmed flu rates during the 2003–2004 flu season, which saw a high number of flu-related deaths in children and, as a result, was a hot topic in the media. During periods of intense media interest or unexpected flu activity, Google Flu Trends might be less accurate in estimating flu rates because of the heightened public interest and increased search activity, the researchers concluded.
"Still, Google Flu Trends provides an excellent public health service, because it's fast and cheap and requires very little infrastructure," Ortiz says.
Google Flu Trends was launched in the U.S. in November 2008. Here, it tracks flu-like illness at the state level, but work is under way to narrow monitoring down to individual cities. The effort has since expanded to track flu-like illness in 20 countries, with results in 38 languages.