Figure 1. United States Influenza Surveillance by Google Flu Trends¹, CDC Influenza-like Illness Surveillance², and CDC Influenza Virologic Surveillance³, June 29, 2003 through May 31, 2008⁴.

¹Google Flu Trends estimates the percentage of persons seeking health care for the non-specific complaint of influenza-like illness (ILI) based on internet key word searches. ²CDC Influenza-like Illness Surveillance involves a network of health care providers who record the weekly proportion of patients seen with ILI. Google Flu Trends was created and validated using CDC ILI Surveillance data, explaining the similarity between the two curves. ³CDC Influenza Virologic Surveillance consists of about 140 laboratories located throughout the United States that report the weekly total specimens tested and laboratory tests positive for influenza virus. This is the only US surveillance system that provides national and regional data of laboratory-confirmed influenza virus infection. ⁴Because CDC surveillance is intensified from calendar week 40 through calendar week 20 of the subsequent year, we restricted our correlation analyses to this time period.