Google Flu Trends has continued to perform remarkably well, and researchers in many countries have confirmed that its ILI estimates are accurate. But the latest US flu season seems to have confounded its algorithms. Its estimate for the Christmas national peak of flu is almost double the CDC’s, and some of its state data show even larger discrepancies.
It is not the first time that a flu season has tripped Google up. In 2009, Flu Trends had to tweak its algorithms after its models badly underestimated ILI in the United States at the start of the H1N1 (swine flu) pandemic — a glitch attributed to changes in people’s search behaviour as a result of the exceptional nature of the pandemic (S. Cook et al. PLoS ONE 6, e23610; 2011).
Google would not comment on thisyear’s difficulties. But several researchers suggest that the problems may be due to widespread media coverage of this year’s severe US flu season, including the declaration of a public-health emergency by New York state last month. The press reports may have triggered many flu-related searches by people who were not ill. Few doubt that Google Flu will bounce back after its models are refined, however.