Comparison of seasonal and epidemic week of onset and peak weeks as measured by Google Flu Trends (GFT) and public health influenza-like illness (ILI) surveillance data at the national (United States), regional (Mid-Atlantic) and local (New York City) levels.

Week of onset was identified as the first of consecutive weeks for each system and region above its Serfling regression 95% threshold, and peaks were identified as the weeks reporting the highest percent-ILI for each season or epidemic. The public health ILI onset and peak weeks are given by surveillance week for each season. The GFT model onset and peak weeks are given relative to the corresponding season/epidemic and regional ILI surveillance weeks.
aOriginal GFT model time periods: The retrospective query selection model-fitting period was from September 28, 2003 through March 17, 2007; the prospective GFT model validation period was from March 18, 2007 through May 17, 2008 and ongoing operation was from May 18, 2008 through Aug 1, 2009. Mid-Atlantic region states included NJ, NY and PA (13). New York comparison was based on NY state GFT estimates (16).
bUpdated GFT model time periods: the retrospective query selection model-fitting period was from September 28, 2003 through September 18, 2009; The prospective operation period has run from September 19, 2009 through March 30, 2013. Mid-Atlantic region states included only NJ and NY (14). The New York level comparisons was based on New York City GFT estimates (16).
**National and Mid-Atlantic region data remained above threshold at the beginning of the 2009/2010 pandemic season.
***No consecutive weeks above threshold to identify onset or peak during this period.