Google Flu Trends and CDC surveillance results were compared for the period of 2003 – 2008. As reported at the 2010 American Thoracic Society Conference, the greatest deviation of Google Flu Trends from CDC surveillance occurred during the 2003-04 influenza season. That year was characterized by early and frequent influenza activity, many pediatric deaths, and heavy focus by the news media.
The main reason for the discrepancy is likely the fact that influenza may only account for 20-70% of influenza-like illnesses. The remainder are caused by other viruses that produce similar clinical syndromes. The authors of the study concluded that “Internet search behavior is likely different during anomalous seasons such as during 2003-4…during periods of intense media interest or unexpected influenza activity such as the 2009 H1N1 influenza pandemic, Google Flu Trends may be least accurate at estimating influenza activity.”
Google Flu Trends does provide an estimate of influenza activity more quickly and cheaply than can be achieved in a diagnostic laboratory. But in this case, cheaper and quicker means less accurate.
Ginsberg, J., Mohebbi, M., Patel, R., Brammer, L., Smolinski, M., & Brilliant, L. (2008). Detecting influenza epidemics using search engine query data Nature, 457 (7232), 1012-1014 DOI: 10.1038/nature07634