Google has produced a special Mexico Flu Trends page to aggregate flu-related search queries from users in Mexico and various states within Mexico.

“We’ve created experimental estimates of flu activity in Mexico using aggregated search data. Unlike Google Flu Trends for U.S., this data has not been validated against confirmed cases of flu. After conferring with US and Mexican health officials, we’ve decided to share these initial results to provide additional information on the evolving epidemic.”

An article in the New York Times, To Aid Mexico, Google Expands Flu Tracking, quotes one expert on the limitations of the Google data

Dr. Henry L. Niman, a biochemist in Pittsburgh who runs Recombinomics, a Web site that tracks the genetics of flu cases worldwide, said that Google’s service appeared to provide only limited advance warning. “I am not saying that it is not useful. It probably works to complement other sources of surveillance and data,” he said.

Google has had a special “flu trends” site up for many months that provides “up-to-date estimates of flu activity in the United States based on aggregated search queries.”

They have found that how many people search for flu-related topics is a leading indicator for reports on how many people actually have flu symptoms. They believe that this metric “may indicate flu activity up to two weeks ahead of traditional flu surveillance systems”. Click on the flash video below to see the relationship between the flu searches and flu symptoms.

So, is Google magic? The explanation for why changes in in the level of flu searches precedes changes in the level of flu symptoms is more mundane.

“So why bother with estimates from aggregated search queries? It turns out that traditional flu surveillance systems take 1-2 weeks to collect and release surveillance data, but Google search queries can be automatically counted very quickly. By making our flu estimates available each day, Google Flu Trends may provide an early-warning system for outbreaks of influenza.”

You can get the details in a recent article in nature:

J. Ginsberg, M. Mohebbi, R. Patel, L. Brammer, M. Smolinski and L. Brilliant, Detecting influenza epidemics using search engine query data, Nature 457, 1012-1014 (19 February 2009).

Of course, such leading indicators may not correlate well if there is a “black swan” flu epidemic or even if there is an unfounded fear of one. Sometimes the crowds are wise, but often not. Remember when we all thought technology stocks real estate was a good thing to invest in?

The Google site also allows you to look at the data by state as well. Click on the image below to try it out.